Tough one...
It was so different back then, in that if you were a 2600 owner, you just hoped for something that was kinda-sorta...there was no point in setting standards any higher than that, given the hardware. To most game designers' credit, they did try to make up for the extreme downgrades from the arcade versions by including several game options (Space Invaders had 16 basic game variations IIRC, but 112 with all of the one and two player options of those 16 variations included).
Anyway, in spite of the obvious limitations, here's what I played the shit out of:
Space Invaders
Pac-Man (yes looking back on it, it was awful...that terribly redundant maze design...)
Asteroids (the more difficult game selections were a pretty solid rendition of the arcade gameplay)
Ms. Pac-Man (absolutely amazing conversion that unfortunately wasn't enough to wash the bad taste of Pac-Man out of players' mouths)
Some other ports were definitely good (Phoenix comes to mind immediately, and Centipede wasn't bad either).
Of course, as everyone here probably knows, part of what helped to usher in the 1983 crash was the fact that a complete shit version of Pac-Man (the one that made it to market was supposed to be a prototype) managed to sell seven million copies (though far short of the 12 million produced)...it was a double-edged sword, in that Atari was convinced that as long as a game had big-name recognition, that aspect alone would sell both the game AND more 2600s no matter what. That led to the double-whammy of Pac-Man and ET (poor Howard Scott Warshaw had just 5 1/2 weeks to develop ET...when you look at the graphics and the animations, regardless of what you think about the quality of the game, it's amazing that Warshaw was able to pull off as much as he did. Given another month, he would've turned in a solid game).