ACK! I just noticed the date of this topic! Since 2004!? Soooo sorry!
LOL No worries!

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The "Best" LBT sequel...now that is a debate that could go on literally until the end of time!
The major difficulty in choosing a best sequel is that the films have made some significant leaps around the "family film" genre. At this point in time, I see 4 (FOUR!) unique groups of LBT films. These groups are as follows:
1) The original "The Land Before Time". This film was released back in 1988 and functions perfectly well as a stand-alone film. What makes this film so different?
Well first, let's look at some of the similarities to the sequels:
It has has a simple plot with some fundamental themes.
It is a unique adventure.
It contains our 5 main characters.
As for the differences, as compared to the sequels:
-The songs are probably the first noticeable difference. Immediately (the opening credits of LBT 2 contains a sing-along), I see the songs as a sign that the audience has shifted from "Everybody" to "a younger audience". This assumption is due to the nature of the songs (very cartoon-sing-along-ish) which almost seem to scream "get your kids to watch me!"
-The original has a terrific musical score which connects the entire film together and keeps it flowing. I almost feel that LBT 1 can relate to Fantasia in a way; every track on the LBT soundtrack, if played sequencially in the proper track order, can almost tell the entire story of LBT 1. The sequels tend to have somewhat more "broken" chunks of music, depending on the type of scene that is on-screen.
2) The sequels LBT 2, 3 and 4. I don't see these sequels as being overly different from the original film. Aside from the sing-along-songs, there are not a lot of fundamental differences between these films and the original. Some of those differences include:
-The sing-along songs, as mentioned previously.
-The sub-themes, which are film-dependant (Example, LBT 3: Sharing is good)
-the musical structure is more vague in the sequels than in the original, seemingly "pasted together" in a way.
-Some of the voice actors have changed.
There are some clear links between LBT 2, 3 and 4, though admittedly these films do not tend to refer to the original film in any direct way. I am able to feel a connection to the original film, and to other films in this group, through:
-The art style, which reminds me much more of LBT 1 than of the later sequels (due to the limited use of computer animation).
-The strong musical links to LBT 1. Many of the same musical themes and sections of the original soundtrack are used in each of the films in this group.
-The characters sound and act in a reasonable manner based on how they are protrayed in LBT 1. I feel that LBT 2, 3 and 4 were much truer to the original characters than the later sequels were. The characters' emotions are more subtle and complex (Cera especially) than the later sequels. While it is certainly true that complex emotional situations exist in later sequels (Littlefoot decision regarding who to live with, in LBT 10, for example) I felt that LBT 2, 3 and 4 were more subtle with complex emotions.
Example, LBT 2: Cera's relationship with Chomper isn't incredibly clear until Chomper is in real danger...It is at this moment that Cera finally starts so show her true concern for his well-being ("Chomper, stay here!").
-LBT 2, 3 and 4 tended to be more conservative with story ideas and special effects, and more subtle with character emotions.
LBT 2, 3 and 4 seemed to understand how to depict a young character's behaviour, and they handled it very differently from the later sequels. This is somewhat difficult to explain, but I'll try:
Let's look at some incidents in LBT 2. We see Littlefoot experimenting with pushing over a small tree after watching his Grandpa do the same to a much larger tree. A simple, totally acceptable childhood situation. Later on, the Gang is trying to reason out what happened to the egg, making some interesting conclusions. This I think was extremely well-done; the gang is still quite young and inaccurate conclusions are a fact of life, especially when only at the start of learning about the world. What exactly happened here...Littlefoot made the assumption that the egg got "smooshed" (spelling?

). The gang then discovers another egg and become so hopeful that they found it, making innocent conclusions that the egg grew instead of considering the fact that it may have been a different egg. Later on in the episode, the Gang talks about parenting the unborn hatchling, talking about how they'll let it do anything (a dream for any child, I'm sure!). There are plenty more examples such as these. LBT 3: Ducky: "Maybe (the thundering falls) is tired and needs to rest?" followed by a *shrug* from Littlefoot, as if he's actually considering that idea as a possibility. LBT 4: Littlefoot playfully amusing himself with the insect, early on the film. These little details make the characters seem more innocent than the later sequels.
The fundamentals of the LBT characters in this group of sequels seem to be innocence and learning to understand life in general. The gang learns many life lessons in these 3 sequels, as they do in future sequels.
3) The sequels LBT 5, 6, 7, 8, 9 and 10.
I consider these almost to be the gang's "Later years". There are some fundamental differences in the entire presentation of the series in this section of the LBT series:
-The most obvious change is probably the new look of the films: More computers, more special effects. The artwork is vivid and exciting, quite a bit less conservative than the previous LBT films.
-The next most obvious changes are the character's voices. By LBT 5, none of the original voice actors for the Gang are still present.
-Slowly, the music in the series starts to develop its own "LBT Series" style, as opposed to borrowing from the original film. By LBT 9 (LBT 8 still had traces), there is literally no musical connection to the first film, yet the music relates very closely to what we had heard in previous sequels. The only major difference is that the music from the Original LBT sountrack is being phased out.
-The Gang seems to act as though they're somewhat older than they were in the previous group of films. The characters are developing less subtle and even
contradictory personalities. Again, this is difficult to explain but I shall try:
From the moment we see the new Gang in LBT 5, they are arguing like pre-teens. "It's my turn to get the first treestar!" Littlefoot Clearly either wants to follow the rules they obviously set (taking turns getting the first treestar? *shrug*) or just wants the first treestar for himself...Now it's debateable whether this action is more or less mature than how he would have acted in a previous sequel, since it is showing that Littlefoot is feeling more confident in himself and therefore more independant. However arguing over such a small fact is really not worth arguing over, especially when they're Surrounded by food. But anyways, that's not where I'm going with this discussion. My point lies in the next scenes where we see Spike willing to knock his friend Petrie out of the sky for the sake of food, Cera willing to chase down a Single treestar, with Ducky practically holding on for dear life...yet Cera says nothing but "hey, that tickles!" Soon after, we see Littlefoot complaining to his Grandpa that "These guys" prevented him from eating the only treestar. PLUS, Littlefoot is resentful towards Mr. Clubtail for simply eating a treestar.
It appears to me that Littlefoot and Cera especially, protrayed behaviours that would have been contradictory to how they would have handled such a situation in a previous sequel. I will admit, the audience does not know the backstory behind why this treestar on this particular tree is so important but I do not think that excuses such behaviour. Though I will say, Littlefoot is not really known to act like this again, so maybe he was just having a bad day.
This, of course, is not the only example, merely the first one that appears in this group of films. Other examples of "excessive personality" include:
Excessive Cowardice: LBT 5: Petrie unwilling to fly across Big Water to save the gang from the island. It's Water. Petrie is in the Air. They just walked within inches of it, on a Loooooong pathway to get to the island. I fail to see the problem.
Excessive obliviousness: LBT 5: Spike: Completely fails to see Ducky before taking a gulp of treestars and nearly eats her!
Mr. Threehorn seems to have become a little less charismatic and less genuinely "Threehornish" between LBT Sequel groups 2 and 3. He also appears to act somewhat less mature. Let's take a quick look at him.
LBT 3: He takes the role of leader. He becomes power-hungry but remains confident even in the most troubling times ("There is no fair when it comes to survival!") He doesn't always make the best decisions but he certainly stands by them. Mr. Threehorn apparently doesn't have confidence in longnecks to lead a herd in a 'real' trouble situation so he puts himself in charge. Seems very Threehorn-ish to me.
LBT 5: He quickly loses intensity and his responses become something that appeals to perhaps a young child's vision of him: "I have to blame somebody...don't I?" He seemed unsure of himself for the first time ever. This response just seemed so unrealistically immature. Cera has been in danger before so I don't think that is the only excuse. Perhaps the fact that he is tired, hungry and thirsty also comes into play.
LBT 6: Mr. Threehorn is low on confidence again! The studdering from Mr. Threehorn is Definitely a new addition to the series as of LBT 5. "Iii..I...It's the time of the changing treestars!" I don't know why the writers made him studder so much...To me, it makes his character lose intensity.
LBT 7: This proves Mr. Threehorn to be Extremely closed-minded, when he stomps his foot and explains to everyone how "Things work in these parts". While Mr. Threehorn was definitely closed-minded in the past, it appeared to be that he was looking out for the wellbeing of the herd or his daughter. In this case, it sounds more like Mr. Threehorn is ranting more than anything. He is simply unwilling to listen to anyone else. "Are there no mysteries left for you?" "Uhh....No!" I honestly don't know how even the 'best' Threehorn could claim to know everything about the universe. Once again, immaturity showing its ugly head.
LBT 8: Mr. Threehorn seemed fairly 'normal' in this film, actually. He was stern and typically protective. My only complaint was that he showed immaturity once again, with his sarcastic remark "Oh, I'm sure he was a big help!" referring to Mr. Thicknose.
LBT 9: Mr. Threehorn once again seemed like his good ol' self in this one. Nothing is coming to mind for me to complain about in LBT 9.
These sorts of distortions to characters are apparent in every member of the LBT cast. Every character, even characters like Grandma longneck, seem to be turning into simpler versions of their former selves in this section of the LBT series. Grandma longneck has hardly had any part of a plot since LBT 6, when she was against Grandpa's telling of the story of the Lone Dinosaur to the Gang. Her voice actor is considerably different as well; she sounds so scripted and un-natural to my ears. In earlier sequels, Grandma really sounded like a Grandma does in real life. her lines have become so generic (""Of could it will, it always does", "No, Littlefoot, not yet."). Grandpa longneck seems to be relatively unchanged, still willing to give advice and listen.
As for changes to the Gang:
As of LBT 5, they have picked up some strange new words: "Fuzzily buzzily" and the like have been popping up in songs and conversation from time to time, when such words never existed in previous sequels. This shows to me more confirmation that the audience of the series is changing from "Young audiences" to "even younger audiences" in a way. Perhaps not extremely (as the themes of many episodes still have complex elements, especially LBT 8 and 10) but there have been some very childish editions to the series in this section of films. Overall, the characters' personalities have become a bit more cut-out and a little less subtle than sequels in the previous section.
4) The sequels 11 and 12.
The first difference I noticed between LBT 10 and 11 was actually the DVD menu music: It didn't consist of either generic background music normally previously found in LBT, nor did it consist of part of a song from the film. It contained music that most closely relates to the music heard in the TV series: a sort of music Never previously heard in an LBT movie except for in the songs. I see LBT 11 and 12 as both a musical transition as well as a general style transition. As of LBT 11, the box art for the DVDs changed significantly...I still don't know why. Unfortunately, it seems to be trying to target a still younger audience. LBT 11 is all about cute little micro-dinosaurs who end up defeating a sharptooth by tickling it. LBT 12 is equally cute, with many tiny flyers, and Tricia. LBT 11 seems to be another transition, for a new generation of LBT fans, for lack of a better explaination at this point.
My conclusion? The LBT series has drifted around so much, that picking a "best" is like comparing apples to oranges. My pick? Anything in sections 1 or 2.