For me, it was the game Rocket Jockey released by Sega Soft in '96. It was perfect. The game featured a surf rock soundtrack by Dick Dale. The entire premise of the game was to ride around on rockets and using long cables to yank your opponents off theirs. Referees were also fair play. I don’t think I’ve met anyone else who has played the game, so if you have, let me know!
Rebel Assault 1 and 2 – they’re total crap, but if you were a Star Wars fan at the time, they were epic. FMV in RA2 was so good, and introduced story unique to the game.
Descent is really hard to beat for me. I put it above Doom/Quake in terms of actual gameplay. It felt so nice to rush to the exit when you’re really good. Only Majora’s Mask 3-days mechanic has rivaled the anxiety factor of Descent.
Heavy Gear > Mechwarrior.
Starcraft/Warcraft.
Heroes of Might and Magic 1-3.
A shit ton of flash games I can’t remember now.