while ive been know to play a myriad of games CS:S is the one that i always go back to... Counterstrike source is one of the most playable consistant FPS out there. the fact that they update it every few years and that you can dedicate and customize your servers is also a great feature. idk of any other game that you can easily design your own maps...while battlefield 2 is great not too many people own a PC that runs it proficiently. Counterstrike is the most consistant and one of the most played FPS for PC games around... even it being an "old game" its still like the 5th most played game on steam. anyways if you havent played it i suggest you check it out, there is a bit of a learning curve cause of the abundance of maps but if you can find a good server this game still cant be beat. also they are beta testing Counterstrike "PRO" atm, cant wait for it to be released!

here is a link to the beta info page

Counter-Strike: Source Beta on Steam