1st - Pitfall or Frogger (I'm not sure which) for Atari 2600 on a console, and the original King's Quest for the PC (played on an IBM PS/2 with something like a 3 or 4 mhz 8086 cpu and a 10 or 20 mb HD, if I am remembering correctly-I was very young back then)
Best - I can't pick just one. HL2 for FPS action, the original Panzer General or the original Civilization and Civ. II for some strategy, and the original Command & Conquer or Warcraft II for some RTS action. Then of course there's Diablo II as well...ahhhhh, too many...I just can't decide!
Future - Nothing in the pipeline really has me overly excited at this point, maybe Starcraft 2 if they don't ruin the franchise with it.