I like Stay Frosty (a homebrew for the Atari 2600)
It's available to play online for free...

http://www.spiceware.org/atari_stay_frosty.html

But I like playing it on a real 2600...




Of course, Mean Santa (Atari 2600) is my favorite...
http://2600connection.atari.org/

but I'm biased because was one of the designers of the game.