Wow dude you couldn't be more off base. Toy Story 3 is considered one of their best movies and considering it's a sequel that's impressive. Both Cars 1 and 2 are considered some of Pixar's worst movies easily (which I still enjoy them somewhat which is a testament to how talented Pixar is). That's not even my personal opinion that's a general consensus.
Wall-E and UP are Pixar's 2 best movies imo and yet they are easily the worst fit for KH3. Fast paced action does not fit those movies at all. As for Tron and Pirates the only reason they didn't "fit" is because they are live action. Not because they aren't appropriate genres.
I do think Inside Out would make for the most interesting one story wise, because you could have the heartless/Org 13 tampering with the emotions of the person you're in.
Regardless Toy Story is a solid pick and one most people are entirely satisfied with. It's also Pixar's longest running and most recognizable franchise other than maybe Finding Nemo.