PLANETSIDE. The only MMOFPS ever created (maybe Huxley will also be this). When it first came out, I had a tear in my eye due to the utter epicness of the battles. Hundreds of soldiers in the air, on the ground, in vehicles, and assaulting via airdrop against defenders of bases scattered on multiple continents. I say "when it first came out", because although it still exists, its just a shadow of its former self.
I miss Planetside so badly. I went back and played a little during the most recent merge and had a blast (especially with the new Engineer goodies), but it's just not the same as it was. It used to be possible to actually fight effectively as an infil, but no more.
I'll never forget fighting for /hours/ over a single bridge, the air thick with 75mm shells (way back before Core Combat) as infantry and vehicles charged across, only to be turned back, with flanking attacks just barely falling short of breaking the bridgeheads... and that one time I killed about eight people in a row by running a Galaxy low over a ridge.
Ah, well. You know what they say about all good things. For now I'm playing Team Fortress 2 and a little DoD:Source shooter-wise (and that's really about all I have time for, beyond MechWarrior 4--at least I'm making good money

).