I've seen every Pixar movie up until WALL-E, since then, with the exception of TS3, BRAVE, and FINDING DORY, I haven't seen any of the more recent ones.
Personally, I really don't feel like Pixar ever missed a beat with their movies (though, in retrospect, A BUG'S LIFE is their weakest, IMO) - each one brings something unique and different to the plate that helps them stand out from the others: MONSTER'S, INC. has an engaging story that satirizes the childhood notion of monsters in the closet as part of their corporate world; FINDING NEMO and WALL-E have some absolutely gorgeous, eye-candy animation; THE INCREDIBLES gives an insightful look into the world of superheroes (and villains), not to mention that both it and RATATOUILLE have ever-so-slightly more mature, almost Don Bluth-esque stories with subtext to them.
It's really hard to say just which of their movies is the greatest, because they really hit some homeruns with so many of them, I really can't remember ever being disappointed with the ones I've seen. I still feel bad about not seeing UP!, but that turned out to be one of those movies where I missed out on a number of opportunities to see it in theaters that it eventually disappeared, and once it came out on DVD and later cable, I lost interest altogether.