For me personally, it’d be things that weren’t just extensions of previous ones – which would rule out Doom or counter-strike – Doom was very similar in mechanic to Wolfenstein, just had a much much better graphical engine, counter-strike was just another multiplayer fps, that happened to be jolly good.
Spacewar, I played a DOS port of it a lot, but it was one of the original games, and I’m so has to be up there.
GTA 1 – first really large scale explory game with a real narrative that I was aware of, and absolutely brilliant multiplayer to boot – just building such a big game, without railroading you into a particular story at every point was a brilliant thing. (Elite I guess was larger scale, but then the large scaleness of Elite was largely repetition and automatic generation, and the narrative wasn’t really there in Elite)
Tetris (Gameboy version obviously – way better than the first PC version). The first computer game I can remember ever being a real sensation outside the world of people who played computer games.
Oh, and not really important, but kind of meaningful for me – that skiing game where you had to stay within the lines (that I typed in from somewhere in Basic on a Casio PB-something pocket computer), and that game at the Science museum’s computer gallery where you have to shoot something (or maybe it is golf, can’t remember), using an old vector graphics computer system controlled by a jog wheel. Oh, and Donkey Kong handheld (game + watch). And Arcade Volleyball, what a game!