Here's what my list looked like:
1.Adventure - just enough complexity to make for a fun little adventure finding items, escaping dragons and getting to the castle. Really unique for the system.
2.Phoenix - Had a friendly competition with singlebanana on this one years ago and got hooked. Had never played it prior to that and he turned me into it. Definitely consider it a must own on the system now.
3. Kaboom! - Just a fun reflex reaction game. Looks like a precursor to a high level Guitar Hero style game when watching others play.
4. Frostbite - My first ever video game as a kid. Takes a little inspiration from Frogger and Q Bert to create it's own puzzle platforming game.
5. Pole Position - Rad Racer style game that was pretty darn impressive on the 2600
6. H.E.R.O. - cool little exploration action game
7. Moon Patrol - that dinosaur game when your browser can't connect to the internet, but still better than that in every way
8. Yar's Revenge - simple concept shmup game in the vein of Galaga, but with its own unique style.
9. Pitfall! - such a vibrant and interesting game on the 2600. Very impressive for its time.
10. River Raid - my favorite pre NES scrolling shmup.
11. Star Wars Return of the Jedi Death Star Battle - a little bit Defender a little bit Yar's Revenge with a great Start Wars license thrown on top
12. Spider-Man - takes the cool swinging from Pitfall and lets you scale building with it!
13. Beam Rider - an interesting take on the Galaga formula that locks you and your enemies onto a set of rails you can transfer between.
14. Star Wars Empire Strikes Back - this is one of my favorite "Defender-clones". What better theme for that style of game than the battle of Hoth.
15. Star Wars Arcade - pretty impressive for the hardware port of the arcade classic
16. Flash Gordon - a little more complex version of Defender with a cool theme attached to it.
17. Missile Command - another ok port of the arcade game.
18. Dig Dug - Same as DK, a passable port, but great memories attached to it.
19. Donkey Kong - This is a big nostalgia pick. There are much better versions of this game to play, but not knowing any better as a kid it was a lot of fun.
20. Super Breakout - pretty slow and painful to go back to now, but growing up was a lot of fun