For me, the definition of "most epic" means that one, the story where the most characters are involved on the largest scale, and two, the one that adds the biggest changes to the story.
So considering that, I'll go with LBT 5 as the most epic. It was the first movie where the Great Valley, the paradise once thought to never run out of food or water, is left barren, and the characters have to leave, wonder around in the Mysterious Beyond, and are threatened with splitting up into separate herds, much like how they were in the beginning of the original movie.
Then the kids discover the sea-well, the Big Water, the Mysterious Island, and a swimming sharptooth for the first time. Then they reunite with Chomper-the only guest character to reappear in the films excluding cameos. Then some other exciting things happen, which while not new, are still quite well-done (the chase scene and the fight scene in the climax). At the end, the gang becomes friends with adult sharpteeth for the first time.
So even though things mostly went back to normal after this, overall, I think movie 5 was the one that introduced the most new elements involving the whole Great Valley herd so it is the most epic.