Okay lets run thru the list:
Toy Story - golden yes 100%
Bug's Life - I could see it working if a good effort was put in
Toy Story 2 - would work as good as, if not better than the first one
Monster's Inc - same thing as Bug's Life
Finding Nemo - I'm gonna say no to the game play, slight lean to yes for the story
The Incredibles - Yes, but now we have Big Hero 6 and I'd honestly be perfectly happy with that
Cars - NOPE BYE
Ratatouille - I REALLY can't see this working that well, would we be fighting food enemies???
Wall-E - Good summon, not a great world, i feel like it'd be the same as Deep Space
Up - that'd be cool just cuz it's Up, but really don't see it happening or working too well
Toy Story 3 - I'd rather have either of the other 2 movies first and we probably will
Cars 2 -
Brave - eh
Monster's University - if we're getting a Monster's anything in this game, summon or world, let's go with the first film
Now the only other pixar films you could maybe consider are Inside Out and The Good Dinosaur, which come out next year, but I doubt anything on either of those from what I've seen on them I think the better choices are from Pixar classics