Here's what I've played so far:
Donkey Kong - fiendishly difficult! Joust - really decent port. Can't get past the first pterry stage, though. Pac-Man - again, great port!
Blue Max - heard rumours that it's a tough game, but I didn't have a hard time figuring it out.
Pengo - didn't spend a lot of time with it, but I didn't mind it. Need to figure out the controls a bit better.
Atlantis - very enjoyable. Spent more time with this than any other game.
Beam Rider - my friends really like this one, but it didn't do much for me.
Submarine Commander - basically put it in long enough to test it. Not a big fan of this style of game, usually.
Final Orbit - again, a game my friends enjoy but didn't do much for me.
Still have to try Missile Command, but if it's anything like the 2600 version, I'll enjoy it.
Thanks for the recommendations thus far!