Of all time? I think Perfect Dark on the N64 (or at least thats the one I have logged the most hrs on :P).
After than - Red Faction 1 for its GeoMode.
if i had to pick 1 right now it would be unreal tournament 2004
i thought girls wont play this genre games B)E)
COD, Medal of Honor, killzone, battlefield
I believe Borderlands counts.
black ops and MW2