I bought my first 2600er back in 1982 mostly because I was impressed by almost ANY of its games!
A couple of friends and me were so impressed by that "magic machine" that we founded a club to promote playing the games together in small competitions with up to 10 players. It was the first Lower Saxony club listet at the German ATARI fan clubs list, the 2nd ever in Germany. And it still exists, 25 years after!
Okay then, the top 5 games that impressed me the most were:
1. Space Invaders (yep - still my favourite after all those years! )
2. Superman (maybe the most innovative game for the 2600er ever!)
3. Adventure (the great-grandfather of all adventure games!)
4. Donkey Kong (I beaceme an addict to that game from the first time I saw it)
5. Dream Flight (IMHO the best shoot-em-up after Space Invaders, but it's THAT rare, it isn't even listet on atariage.com! I can't remember the label, but it was some low budget label late in the 80's. I bought the game at a wholesale just because it was cheap: 10,- DM = about $4,- and I loved it. It was a great mix of Space Invaders, Galaxians, Circus and a whole bunch of innovative, new, crazy ideas. Shame I sold it with my first 2600er a few years after!