I really like most of these movies. Haven't seen Cars 2 (not excited to at all) and I thought Brave was pretty below average and felt like two different movies slapped together. Hope Pixar returns to form before too long. Would rank the rest like this, from best to least good:
Toy Story 3 - Having watched Toy Story as a kid and being a man too immature to throw away his toys, this movie hit really, really hard. If you're like me at all, either the end of this movie made you tear up or you are a liar. First half is less good, but still a highly entertaining prison break film.
The Incredibles - One of the most significant animated films of its time, with a great message not everyone is comfortable hearing. All-around, a killer action movie. Watch the director commentary if you have the DVD; it's one of the best I have heard, especially Brad Bird's thoughts about animation as a medium.
Up - The talking dog stuff is a little silly, but otherwise this is a grand, beautifully realized film. The first ten minutes are, as everyone knows, jaw-dropping, but I really liked the odd couple relationship between the main characters and thought the villain did a good job showing the dark side of committing to a lifelong dream like the main character did in a way that didn't feel ham-fisted.
Toy Story - Gets a lot of slack for being their first, but honestly it holds up great considering it's basically the first of its kind. Smart characters, well-written and with snappy dialogue. Woody and Buzz are unforgettable.
WALL-E - Another master class Pixar movie. Gorgeous space scenery and an oh-so-cool avoidance of dialogue for a long period of time. The message of the film is presented in a way that rubs people the wrong way, but I think it's largely because it makes them uncomfortable to hear it. (not saying I don't think it's a little unsubtle, but this didn't drag the movie down much for me)
Ratatouille - I don't consider this quite on the tier of the above films, but it's definitely next in line. Patton Oswalt makes a fine performance, and the human animation is some of the best Pixar has done. I didn't think much of the critic (and I'll admit the ending didn't have any buildup and kind of lost me) but I understand why people like his character so much. This film feels very tightly designed.
Toy Story 2 - Not sure why critics think this is so much better than the original; I certainly don't think it is, but it's a great time regardless. It's the funniest of the trilogy, but tugs at the heart strings a little too. I don't care for Jesse the cowgirl (she's honestly a little annoying) but I enjoyed the conflict Woody goes through enough to make up for that. For me, the side characters steal the show in this one (Buzz, the other Buzz, Rex, Potato Head, etc).
Monsters, Inc. - A lot of people seem to favor this one highly, but I just think it's pretty good. Maybe the cutest Pixar movie, though. The monster designs are a lot of fun.
A Bug's Life - After watching Seven Samurai and a few other samurai films, I got a kick out of all the references to them here. It still isn't as strong as some of Pixar's other material, but it's very watchable.
Cars - I don't hate Cars, but I didn't like it enough to watch the sequel when it came out. It's kind of a generic story, but it's pulled off all right I guess. Probably a bigger hit with kids who are still in to Hot Wheels and all that, so I'm not going to hold my lack of caring for automobiles against the movie.