Well, if you're limiting yourself to Atari-developed games only and nothing that ever appeared on another platform, then you're really cutting down your options. Atari didn't actually make that many original, unique-to-the-2600 games. In fact, a quick search of MobyGames (a pretty good - though not 100% complete - source for listings of home console games) only lists 114 games developed by Atari itself for the 2600 (101 are listed under "Midway Games West" which is Atari's current name, and 13 others are listed under "Atari Corporation"). 36 of them were arcade ports, so right away you're down to only 78 options out of the hundreds of 2600 games. And a lot of those 78 are generic games like Hangman and Checkers, educational games, primitive puzzle games, and first generation sports games that you'd probably find more frustrating than fun. So if you strip away all of those, as well as E.T. (one of the worst video games ever made) and anything that was ever re-released on a superior platform, here's the 25 games on Moby's list that are left:
Adventure, Air-Sea Battle, Circus Atari, Combat, Dark Chambers, Demons to Diamonds, Haunted House, Human Cannonball, Maze Craze, Obelix, Quadrun, Radar Lock, Raiders of the Lost Ark, Slot Racers, Snoopy and the Red Baron, Solaris, Sorcerer's Apprentice, Star Ship, Street Racer, Superman, the SwordQuest series (3 games), Taz, and Yar's Revenge
Out of those, the games with the best reputations besides Adventure are probably Combat, Haunted House, Solaris, and Yar's Revenge. I haven't played Combat, and it's a 2-player only game anyway so I don't know that it would be much good to you. I have played the other three, however. I thought Solaris and Yar's Revenge were both a lot of fun, and I'd recommend both, especially Solaris if you like long games. Haunted House looked cool but I could never figure out how to play it, probably because I didn't have an instruction manual.
Now if you're willing to open up your developers a little bit, Activision and Parker Brothers both made some games for the 2600 that were pretty fun. Not even mentioning classics like Pitfall and River Raid that again also made it to other platforms, they still made some pretty solid 2600-only games like Chopper Command, Spider-Man, and Star Wars: Empire Strikes Back, all of which I had and enjoyed quite a bit.