Don't know if you mean "Best" as in actual production values or "best" as in what's a fun awesome movie they'd enjoy. I love and have shown my kids a ton of them and here are just some cool ones that I can think of and vary according to age.
Any Disney Cartoon movie (Lion King and Aladdin are my favorites)
Most if not all Pixar movies (I really like A Bug's Life too for some reason)
The animated Hobbit from the late 70's
The Flight of Dragons (I think from the late 70's as well)
The Lion, the Witch, and the Wardrobe (animated 70's cartoon that is very well done just NOT the weird live action BBC series)
How to Train your Dragon movies
Land Before Time (JUST THE FIRST ONE)
An American Tail
The Secret of Nimh
The Prince of Egypt
Transformers the Movie
GI Joe: The Movie
Shrek movies
Secret World of Arriety
I think that's all I can think of right now.