I enjoyed a lot already mentioned like Kaboom, Circus Atari, Yars Revenge, Pitfall, etc. Had forgotten completely about Freeway and Barnstorming...yeah, they were fun too.
I think I remember finishing Pitfall. The game ended with like 14 seconds remaining, or something? I dunno.
I recently tried downloading a Kaboom app for the phone, but didn't find anything impressive.
I never got a 2600 emulator, so from memory I recall loving Maze Craze and look forward to trying it again someday. Also liked Dodge Em, Sky Diver (splat), Football, Baseball, Breakout and the Sprint racing. Sorry, think I probably have the name of that one wrong, but I liked the ice track - one bad move and you were in the wall.