Counter Strike series, source specifically. I love the simplicity and precise nature of the "base" game, but the modding scene is where the game came into its own for me. Countless hours spent on trying to perfect speedy runs on surf maps, dealing with almost unbearable levels of ping just trying to play on a US jailbreak server (awesome community compared to the UK scene at the time so it was worth it), endless amounts of custom games, and basically everything else (deathrun, mini games, etc.)
And no, I haven't played "properly" in ages. I booted it up some time ago just to check things out, and it was somewhat sad to see how everything had moved on. Servers, dead. Players, either "dead" or move onto something else (well, like I did.)