Monsters, Inc. (voted for) I really liked all of them but I would say Monsters, Inc. just because it is the one I am most emotionally connected to. Toy Story is in a close second. I haven't seen Up YET
up begins with a little boy name carl who likes adventures. he meets a girl, ellie, who also likes adventure and she plans to be an explorer. they eventually get married and carl promises ellie that they will visit paradise falls but ellie gets sick and dies. carl takes a bunch of balloons and flies to the falls and... read here for more http://en.wikipedia.org/wiki/Up_(2009_film) as you can see, im pretty bad at summarizing