To be honest, I'm not really a fan of playing "favorites" when it comes to something like this.
Neither am I. I rarely use superlatives. I hope it is okay when instead of numbers I give a few words on the sequels which may tell more than numbers can.
I was rather fond of LBT 2 (still am). While the movie clearly broke with traditions of the original movie (befriending Sharpteeth would have been of limits in the land of the original movie) and also had a few changes in style (Chomper came as the first dinosaur in a color really far from the colors usually associated with dinosaurs). I like the story though. With the eggeaters we had some villains for the first time whose hunger was not the only thing to label them as villains. While LBT never made to really deal with the problem of a Sharptooth leafeater relationship this movie abandoned a kind of black and white / good and evil view of which the original movie might be accused. It was a bit difficult to get used to the idea of singing dinosaurs. None of the LBT 2 songs ranks among my favorites, but neither do I have a particular dislike of any of them. LBT 2 had a lot of the kind of sentimentalism which I suppose contributed a good deal to making me an LBT fan.
Many people don't think very high of LBT 3. Perhaps we are a little harsh on the movie. LBT 3 had a solid plot and some really interesting ideas. The major problem of the movie was that it was too moralizing, especially towards the end. If they could have brought their messages across a bit more subtle it would have been better. Still the movie had many aspects of a good land before time story.
LBT 4 brought us the character who was possibly the one most frequently discussed in the forums. The promise of Ali's return used to be a straw for LBT fans to cling to in times when it didn't look like there was going to be a next movie. Meanwhile a return of Ali was shown. I did not watch it as I heard of very negative reactions to the respective TV episode. I fear that it might do more harm than good. Anyway LBT 4 had a solid plot and the messages were brought across in a better way than was the case in LBT 3 (perhaps because the LBT characters didn't come across as faultless in this one). Spike's calling of Ducky was a very memorable scene. While I cannot name one favorite sequel LBT 4 is certainly one of those I am really very fond of.
The same goes for LBT 5. At long last we had at least a bit continuity through the return of a character. The reference to Littlefoot's Mum in the song Always there is another element which secured this sequel a special place in my heart. It had a good story and beautiful elements. However, the first LBT sequel produced by Charles Grosvenor also gave a glimpse at some of the elements which I really dislike in his movies. Why on earth does everything have to turn red in case of danger? That was really not necessary. Logic failures such as a tsunami rolling from the shore towards the open sea or lack of explanations (how did the gang get onto that log after they rolled it over the cliff?) did not occur in Roy Alan Smith's movies.
LBT 6 is another one of the sequels which some people don't like while I don't have a major issue with it. It is very regretable that interesting characters such as Dinah and Dana disappear after a single sequel without any further reference in later movies. LBT 6 brought new elements to the land before time. Unlike some later sequels LBT 6 did not spoil the issue by disenchanting the mysterious. At the end of LBT 6 everyone could decide for him or herself whether or not to believe in the lone dinosaur legend.
LBT 7 made exactly the mistake which LBT 6 had avoided. They disenchanted the mysterious and brought an element (aliens from outer space) into the land before time which does not belong there. There were also some really annoying shortcomings in the animation (anyone seen green lava before?). This was the first land before time movie I felt uneasy about. But with all these drawbacks LBT 7 also had some positive aspects. Pterano is one of the most interesting guest characters ever. The story of what happened during the journey to the Great Valley also reminded us of the gloomier elements of the land before time (not just a funny playground for dinosaurs). The song "Beyond the Mysterious Beyond" was quite beautiful and "Very important creature" gave a very good insight into Pterano's way of thinking. But again there were these sadly unrealistic elements which I'm sure would not have occured in a sequel directed by Smith. Using a hot spring as an elevator, Littlefoot, Cera, and Spike riding on the back of a single flyer this is really sailing close to the wind.