I'm going to start with the funniest Disney film ever for me; The Emperor's New Groove - it's just snappy, snappy, snappy humour all the way through with, quite ironically I feel for a Disney movie, no stand out musical number which stops it being perfect.
Kids films generally?
The Princess Bride is just a wonderful, wonderful movie which I didn't see as a child but so wished I had.
In the same vein is Back to the Future - Marty McFly to any 8 year old boy would be the single most awesome guy in the world.
Fantastic Mr Fox is probably one of my very favourite films ever!
EDIT: Totoro is another amazing call. Nothing happens in it - and yet it's so... so... satisfying.
In that same sort of area, Where the Wild Things Are is phenomenal but I don't know how much kids would 'get' it. I figure though, if they can get Totoro then they can get WTWTA.