Team Fortress 2!
'nuff said. It's here, it's new, it's cool. And i played it. You can look it all up on my TF2 stats page!
My fav is the engineer so far, alltough this class is already so widely played, i try to avoid it nowadays. My sympathy goes for the spy now, as everybody hates the spy (including other spys). For understandable reasons i have to add, as he is a pain in the ass for almost every class (maybe excluding the scout, as this one is simply too fast to kill in melee). Especially thankful targets are of course snipers, as they present a perfect aim for backstabbing, when they are trying to get the next headshot. And your ability to sabotage the funky toys of the engineers makes them enormously annoyed and suspicious about practically everyone. If you get shot on by a friendly engineer, don't mind, he is surely just very caring about his turret or teleporter or whatever he calls his child at the moment.
But of course the most mentionable about the game is the look. It all reminds me so much to roadrunnder-cartoons, that i am constantly laughing just at how all these characters are represented. From the medic as a german doctor ("I am ze Uebermensch!") to the grunty heavy with his minigun fondly called "Sasha"... It feels like you take your part in a roadrunner (which clearly is the scout guy) and coyote (which is approximately every other class) cartoon. That alone is already a huge motivator! As the whole action of any FPS and this one in particular is pretty cartoony with respawning people, unrealisticly huge cannons with unrealistic amounts of ammo and an inhumane playing speed, valve's choice going with such a cartoon style fits just too perfect.
With other words: If you have just the slightest sympathy for online FPS, then you got to buy this.