What I love about Pixar is how they get tremendous performances from people who are TOTALLY new to voice acting. The guy who played Linguini in Ratatouille was what, an animator I think? And he was great, as great as any celebrity actor would've been. Then you've got Sarah Vowell in Incredibles. Yeah, she already has a distinctive voice, but she still took that character of Violet and made it her own. She conveyed that character's insecurity and transformation and blossoming so well, it really won me over on Violet in general.
And Mater is BY FAR the best performance of Larry the Cable Guy's career. I wanted to hate Mater but I just couldn't, Larry did such a great job of delivering those lines and my eyes practically teared up at the "I knewed I made a good choice when I picked you as my friend" lines.
So yeah, Pixar is definitely the best at casting voices for animated feature films. Disney is second best, they've had a few stumbles but have been pitch-perfect for the most part (they knocked Tangled out of the park with Moore, Levi, and Murphy, the PERFECT choices for that roles though I am still a bit curious to hear how Grey Delisle would've done as Gothel).