It's hard to say which one I think is the most epic. To me, epic means how grand/great I thought it was. Going off of that, I'd have to say LBT: Stone of Cold Fire was the best, in my opinion. My reasons behind that decisions?
I found all the songs enjoyable. Especially the 'Beyond the Mysterious Beyond'! It's not as obnoxious as a lot of people claim it is. I love the design and personalities of the rainbow-faces and their other-worldly origins!
Aliens in LBT is kind of silly

but also cool! It kind of adds to the mysticism the sequels gave it. Besides, it had one of the best characters to come from the LBT franchise, Pterano! He's pretty complex ( or as complex a character as these movies can make )and I just loved his character. Plus he's kind of cute for a pteranodon!

Plus Rinkus and Sierra were pretty cool as well.