<blows dust off account> Whoa, this old thing still works. That was a nice little surprise...
Welp, it's been a loooong while since I posted anything here, but after seeing that Jenny Nicholson review I felt the urge to jump back on quickly to post some of my thoughts on the films. Time to quickly pilfer from a ten year old post of mine!

Hmm...Of all the sequels, the one I saw the most as a kid was 4 (only had that one and the original film), and I thought it was awesome for giving Littlefoot a love interest. I know I saw 2 at some point, because I had vague memories of a sharptooth baby.
Then, when LBT 7 came out, they ran an advert for it in a kids magazine I was reading at the time, and I honestly went "Seven? There's seven of these films?". So over the next few days, I went to the video store and borrowed the ones I hadn't seen. 2 I liked, 3 annoyed me (bullies), 5 was great, 6 I got bored watching for some reason. I remember watching 7 and loving it.
The next year 8 came out. This was the last one I saw for a while, because I rented that and thought it was bad. When 9 came out I had stopped caring (I remember perhaps one advert, but aside from that nothing). By the time 10 came out, the adverts were everywhere and I noticed them, but I was also starting to pay attention again because I'd heard on the imdb forums that Littlefoot's father was in this one.
I didn't see it, but when 11 came out on DVD, my dad bought it because it included the first movie with it. I saw it, and thought it simple kids fare (although I despised that Girls and Dads song...that note at the end just sounds off to me). After a while, I wanted to see the films again and catch up, plus I hated the idea of only having "Part 11" on the DVD shelf, so I started tracking down the films from the stores. I got 5, 6 and 10 first, and despite my memories, this time I preferred 6 to 5. 10 was alright, but Littlefoot's voice actor annoyed me so much that I rarely watched this one afterwards.
I think I got the three-pack next, with the first two sequels (I liked 3 better this time). Dad tracked down 12 for me (and I thought it was actually pretty good) and I got 4, 7, 8 & 9 at a Christmas sale. 4 wasn't as good as I remembered, 7 was still great, I liked 8 better this time (although I didn't think Ducky's voice actor sounded right anymore), and 9 was ok. Finally, my Dad found 13 for me. I actually didn't find it that bad (probably because my method of watching these films is to muck around on my computer with the movie playing in the background).
Out of memory, I'd say 5 & 6 are my faves, followed by 7, 12, 2, 3, 8, 4, 9, 13, 11 & 10. I certainly agree with Dino-doug about the animation, I love the way the newer films look.
EDIT: Although, now that I think about it, one thing annoyed me about the DVD for 12; the unskippable adverts at the beginning. When I got it, I sat down to watch the film, and was greeted with 5 minutes of unskippable (although thankfully fast-forwardable) adverts for Barbie. At the time I went "Barbie?! LBT isn't a girls-only franchise!?"
I really should watch some of these films again to see if my opinions on them still hold up...I tended to rank them more on the quality of the songs and the voice acting than the story and visuals, because at that point in time I was using the films to provide ambient noises (as well as a nostalgia buzz, of course) while I was playing games on a nearby computer, but a few of them are on there because I liked the ideas they were playing with.
Anyhoo, let's work backwards up that list, leaving off 14 because I barely remember the film (well, I liked Reba McIntyre's song in 14, but that and Littlefoot's grandmother nearly dying (and the replacement voices for Grandpa Longneck and Daddy Topps not sounding the same at all) are pretty much all I remember). 10's at the bottom mostly because, as I noted above, I couldn't stand Littlefoot's voice actor (his voice was much higher and squeakier than before and it was jarring, plus they re-used his nightmare scream in a later scene and that laziness killed any immersion)...plus it felt like he was oddly juvenile here compared to other installments.
11 and 13...well, these are some of the films I rewatched the least out of them all, so I can't remember exactly what put me off about them. I guess both stories were kind of dumb, and 10 just got ranked lower because of the voice acting issue. 9 is one I don't remember disliking, but I think it never quite appealed to me the same way it did other people on this message board.
I must say I'm surprised at the next ranking, though; 8 better than 4? I mean, I remembered 4 not being as impressive as I thought years later (let's be honest, we all liked it because of Ali, and the hint that she would be important in Littlefoot's future), but 8 was a film I really didn't like when I first saw it (although I mellowed towards it upon rewatching) and which I didn't like the music from at all, so it must have done
something right to get that far!
2 and 3 next makes sense; 2 naturally pales before the original, but it's still a sweet and whimsical little film...a nice epilogue with a far less taxing adventure for our protagonists, plus I have a soft spot for the music in this one. 3 not so much, but I'm guessing there was something I liked about it the last time I watched it.
Now we get into the top ranking films, and these ones are fresher in my memory as a result. 12 was a favourite of mine for a while because I liked all of the songs as well as Guido as a character, plus I remember thinking some of the dialogue was pretty well-written. 7...okay, this one was probably there for nostalgia; the animation in that one is a bit choppy considering it's the transition point between the older hand-drawn style and the digital animation, plus the villain song is the only really good one in it, but the worldbuilding was interesting (insight into what the adults were doing during the events of the first film) and Pterano was a compelling antagonist of sorts. 5 and 6 took the top spots overall, but I'm trying to remember exactly why. I mean, both films have some really good songs in them (ignoring the screechy chorus of Always There, the bit where Littlefoot sadly sings about his mother is very powerful), but I'm not sure I'd call these stories the best of the series. Still, something about them was soothing; something I could put on and lose myself in in a way that the other films didn't quite make me do.
...I probably will need to watch them all again and see what I think.