I'll second Snatch. I've seen it 6-7 times, and could easily see it again and still laugh my ass off.
Honorable mentions:
Braveheart
Saving P.R.
Gladiator
We Were Soldiers
and nearly any other war movie, preferrably WWII.
Worst movie I've seen,
Fast and the Furious... Even if I ignore the technical side of things in that movie, it was still beyond stupid.
-J
