The original Toy Story looks dated in some aspects (especially Sid's dog), and Bug's Life looks very dated in the daytime shots, but other than those, I'd say their 5 film run from TS2 through to Cars still hold up today, aesthetically and narratively. They don't have grossly exaggerated details like their modern contemporaries do.

Ratatouille and to a far greater extent WALL-E is where the obnoxious beanmouthing starts, as well as the more subversive meanings of their later output (i.e. you don't matter goy, you won't achieve your dreams, don't be prejudiced, everyone dies some day) start to shine through. And it's hardly a surprise because that's when Lasseter started becoming less involved and Bird, Stanton and Docter took over. By Up, it's basically a straight line from there to their current movies.

Cars 2, as kid-oriented as it was, was probably the last "fun" Pixar movie that didn't have some sinister subtext to it.