My list:
(not in any particular order)
Half-Life
Half-Life 2
Halo
Halo 2
Grand Theft Auto 3
Grand Theft Auto: Vice City
Grand Theft Auto: San Andreas
Call of Duty
Call of Duty: United Offensive
Doom 1 & 2 (They're Classics
)
Doom 3
FarCry
Medal of Honor: Pacific Assault
Need for Speed: Underground
Need for Speed: Underground 2
Rome: Total War (Massive battle sequences, and I mean MASSIVE)
Splinter Cell
Splinter Cell: Pandora Tommorrow
Splinter Cell: Chaos Theory (Not out yet, but I can tell its going to be awesome)
America's Army (FREE FREE FREE!!!!!)
And some other ones that I forgot about....