Quake II, Half-Life, Quake III, RTCW, RTCW:ET, Tribes, Worms 2, UT, Unreal, Doom, Quake, and Tony Hawk 2.
Out of those, mostly Q2/Q3/RTCW/RTCW:ET. Which is the best? In gameplay, either RTCW:ET or Q3 for different reasons. RTCW:ET is the refined RTCW, and Q3 is the refined Q2. In their most basic forms.. one caters to lovers of slower, more realistic.. mostly squad/team-based combat where each person has a specific role. (RTCW)
The other caters to those with lightning quick reflexes and a hankering for unrealistic carnage. Not to say that mods have been created in both games, in which this basic gameplay is totally scrambled.
For example, my two favorite Q3 mods are very team-oriented.
Enough with the rambling.. which one do I like more? Neither, but I play mostly (if not exclusively) RTCW:ET these days. The servers are full of newbies and the maps are starting to get old.. but I play it daily.