Best for PC (in no particular order)
Doom - definative fps
Deus Ex - still a brilliant game
System Shock 2
Llamatron - maybe just me but a simple brilliantly addictive game.
Half-Life 2 (+ episodes and portal)
Knights Of The Old Republic (KOTOR)
Call of Duty 4
Company of Heroes
They are all great games but if I had to choose one it would have to be Half-Life, the best fps ever imo and still the greatest game made so far. Though I have to admit I'm not a huge RTS or MMORPG (that's too long an acronym anyway
) fan but still....
I don't buy bad games
but if you forced me, the worst I own is Doom3 - the only game I have never finished but to be fair a friend recommended playing it on very hard and it was much better so maybe I'll go back to it.