I've yet to see UP or Ratatouille, but for my money, no production company/creative group has churned out such consistently high-quality movies, animated or otherwise, as Pixar. Of all their offerings that I have seen, only A Bug's Life was a disappointment.
Perhaps it's also the father in me, but Finding Nemo is simply a fantastic movie. It is a great adventure, it is heart-warming, fantastic message, and wonderfully executed story-telling. And as a father, few movies hit me more than watching this with my two boys.
Cars, while not quite as well-rounded, is great fun and hits the mark for it's targeted group, children.
And WALL*E? That is pure awesomeness from the opening scene through the final credits.
I've heard nothing but great things about UP, but I still can't find myself to watch Ratatouille as it just does not look interesting in any manner. But as it is a Pixar movie, I will give it a go eventually. Again, with their batting average, I have to give them a chance every time.
Toy Story = Awesome
A Bugs Life = Really f'ing boring
Toy Story 2 = Fantastic sequel that nearly surpasses the original
Monsters Inc = Awesome with heart (The final "Kitty" gets me every time)
Finding Nemo = See above
The Incredibles = fantastic mash-up of genres
Cars = Good, tiny bit long, but still a winner
Ratatouille = never saw
WALL*E = wonderful
Up = never saw
Seriously, what studio can boast a track record like that?