After I watched Billy Madison with my children for the first time, they both said "now we know where you get 90% of your vocabulary from". LOL!
It's hard to have Billy Madison on the list, but not Happy Gilmore, but I also don't want two Sandler movies on the list...
I also rewatched Superbad recently, and laughed just as hard as before (although I identify more with the cops than I used to).
Spaceballs is another that I have seen countless times, though I'm waffling on including it on my list. I might have to go with Dumb and Dumber and Big Lebowski.