Voted for battlefield --as a series, not just a single game.
BF1942, when it came out, was totally earth shattering. Not only was it an FPS but you had vehicles that you could use to add a whole new aspect to FPS gaming.
I've played Battlefield since BF1942 beta. I actually got booted from tech tv's lan party for pwning too much (30-0, and 3 bases captured within 5 minutes, in a Sherman
). I bet the BF series, especially 1942, has more mods than damn near any game out there. There were Star wars mods (galactic conquest), Vietnam mods (eve of destruction), and probably the most successful mod Desert Combat.
BF Vietnam was kind of a flop IMO. Why would you pay for a game that played worse than a mod (eve of destruction). BF2, was basically a copy of Desert Combat, except done by dice. Except I think that the planes fly like crap compared to the Desert Combat mod.
BF 2142 was really cool. I liked the whole futuristic aspect of it with the battle walkers and hovering tanks.
Except for it being set in the future, the gameplay is identical to BF2.
BFBC2 is growing on me. I absolutely hated it at first due to some features that were removed, such as prone. I also hated the fact that it takes so many dang bullets to drop someone. I still think the kill cam is really lame.
All in all though, the BF series is probably the best out with quite a large following. So it got my vote.
PS: How come you didn't put counterstrike in there? That game, at least for a while, has had the
largest user base since its inception.