Way too many to point out, but some highlights:
Thief: The Metal Age (with it's earlier release in '98, defined and set the bar for the stealth genre)
Black & White (brilliant concept)
STALKER
Doom3
FEAR
Call of Duty 4 (MP experience was a blast)
Team Fortress 2 (absolutely the best MP experience)
Crysis (best playable tech demonstration to date)