The best of the movies, I think, is the last one because it is quite serious and deep and scientific. It rounds of the whole Ben 10 story as it was supposed to be the final part, but everyone hopes it won't be as all the characters and the animation have been so exciting.
When Ben tries to set things right after accidentally setting the deadly Omnitrix to detonate, it is difficult to take a breath. Seeing Ben to track down its creator halfway across the universe if he wants to save the solar system from certain destruction is surely the most gripping movie.