All of them.
Toy Story was the first film I ever saw in the cinema when I was 5 (now films are my obsession - what a great start)
A Bug's Life is great but not their best. Got the DVD for free with a family ticket to Wall-E and you can't say fairer than that.
Toy Story 2 was better than Toy Story 1 IMO
Monsters Inc is so much fun, especially the doors bit, but is one of the few I don't own
The Incredibles I didn't think much of at first, but has grown on me on repeat viewings. I don't own it
Finding Nemo is one of the greatest films of my generation. The animation is incredible
Cars I loved (even though nobody else seemed to) and saw 4 times in the cinema (til Toy Story 3 my personal record), have seen it now a total of 16 times. Can't wait for the sequel, hopefully it won't be a let down. I got obsessed with Mater and own just about everything you can buy, even a Christmas tree bauble from Disneyworld.
Ratatouille I own but I'd say it's probably their weakest but still good
Wall-E is a work of art. End of. AMAZING.
Up was fantastic and very funny. However it is another I don't own. Yet.
Toy Story 3 was the best TS yet. Who else could make grown men cry over pieces of plastic!? I saw it 5 times in the cinema, and have watched it twice on bluray. The ending two scenes still make my spine tingle. I rarely cry, but I was VERY close with this film. I still have to hold my Woody and Buzz toys (I spent £100 on them this year because I never got them when I was 5 - I was 19)
Pixar can't go wrong. I love them.
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Spare a talent for an old ex-leper?
Monty Python's Life of Brian