Bottle Rocket.
I totally buy this. Had never been able to sit through the entire thing before, but was recently sick and watched the whole movie start to finish. Was highly enjoyable (may have been delusional from fever so cannot confirm though).
Beerfest/Super Troopers and Happy Gillmore/Billy Madison get funnier with each watch ... until you hit a wall at around 50 viewings or so. So they qualify to a point.
I'd say my enjoyment of 2001 has roughly doubled with each subsequent viewing, though I hated it to start with so it had a lot of room to grow (now probably in my top 10). The remake of Star Trek was pretty meh on first viewing - much better the second time (though that may not be "old" just yet). And I guess I'll add Wonderboys just to be different. Love that movie more and more with each viewing.