Ahhh....there are so many different ones for various reasons. There are some awesome comedy performances, drama performances, horror performances...I seriously don't think I can name just one...but if I had to...I'd say it's "The Net" with Sandra Bullock - no laughing! I can watch that anytime!
Others I can watch at anytime, no matter what part I happen to pop in on:
It's A Wonderful Life
and probably many more that I have forgotten!