Ooh, wow... that's a good one.
I've got a pretty good idea of the ones in the running:
"Toy Story" - It's the original, so it's got the nostalgia factor, and it's a great story that never gets old.
"Monsters, Inc." - I like pretty much anything with Billy Crystal in it, so Pixar + Billy Crystal = win-win.
"Finding Nemo" - Another great story and, honestly, who really doesn't like this movie?
"WALL-E" - An excellent movie despite the not-so-subtle, global-warming, alarmist overtones.
"Up" - Perhaps the most well-rounded of all of the Pixar movies, which is saying a lot.
If we're going with the movie I would most likely watch the most often, especially when just happening across it while channel-surfing, I guess it would be "Toy Story". However, if you told me I could only pick one of them to ever see again in my lifetime then I would probably have to go with "Up".
So, I guess I'll go with "Up".