Oh boy...
Comedy:
Dumb & Dumber
Half Baked
Billy Madison
Happy Gillmore
Office Space
Big Lebowski
ANY Kevin Smith film (Clerks, Mallrats, Chasing Amy, Dogma)
Friday
Next Friday
Austin Powers 2
Airplane
Any John Hughes film (I mean, come on, we've all seen Breakfast Club 1000 times on TNT)
SciFi:
Matrix
Contact
Gattaca
2001 (but really any Kubrick film)
Drama:
American Beauty
Boogie Nights
Magnolia
Vertigo
Cruel Intentions
Ice Storm
Phenomenon
Usual Suspects
And there are more.... all of these movies are endlessly rewatchable for me.
l2c