I also voted for the cavern from LBT 4 because...
1.) there was quite a lot of action
2.) it's the setting for quite a long time in LBT-terms

3.) Archie has his whole screentime there
4.) there were quite a few strong scenes like Cera returning and later the Gang saying 'goodbye' to Tickles. Ali said a famous quote there:
"Maybe we'll all live together someday!"Some thoughts about the other scenes:
Though Littlefoot and Cera met The Sharptooth in this underground chasm I don't have any positive nor negative points for it! It is a kinda short scene. It's quite funny to watch Littlefoot and Cera popping those bubbles though

This place with the Domeheads in it is relatively high on my list. The tar monster scene is one of the best scenes of the whole franchise and pretty laugh-worthy.
The cave where Ducky was the bait for The Sharptooth is high on my list as well because it's pure action! I guess if you add the outcutted scenes it'd be even longer and better

Nothing special about this cave in LBT 2
I guess it's the same cavern in LBT 7... the cave in which the Gang fell was the exit of the cave Ducky fell in I'd say

I was arguing whether to vote for this one, but I eventually chose LBT 4's cave scene. I like this place because the song 'Good Inside' was sung there which had some pretty funny scenes like Ducky tickling Petrie in it

Threehorn peak... mysteeeeeeeeeerious

greenfood from nowhere and green lava/magma??? I like this place

The cave of the Tinysaurusses wasn't very interesting at all, but it was the setting for quite a few time and some pretty funny scenes like Spike getting stuck and an okay song happened there as well...
I should stop to do such long posts :^.^: