Not many following the rule to pick ONE movie, so i will
Also trying to be very thoughtful about this, because the question is not asking for your FAVORITE movie, just the one you can watch the most often.
With that said, I vote for 40 Year Old Virgin. Because the movie gets funnier every time. It's not even in my top 10 all time movies (comedies can't make that list imo), but it has the rewatchabe quality.
For example, Saving Private Ryan is in my top 5, but that movie is just too hard to watch to want to see it all the time