First:
Lemonade Stand (1979) on the Apple 2.
It's not much of a game, though ... I did remember playing a bunch of games including Ultima, Bandits, Taipan, Zaxxon, Wizardry, Archon ...
Latest:
I have Fallout New Vegas, but haven't installed it yet. So, err ... I guess Puzzle Quest 2.
Notable:
Ultima 4: Quest of the Avatar .. impressed me 'cos it was my first CRPG with real character development. Like whoa, the possibilities.
Planescape: Torment .. 'cos it actually impressed me more than Ultima 4 did.
Competetive gaming scene over the decades ... from Street Fighter 2 (arcades of course) to Counterstrike to Starcraft. Starcraft community in particular, are just damn cool ... total opposites to the TOOLS on XBLA.