Curiously, despite the fact that I'd HAVE to say Dota2, I can't deny that the FIRST game to come to my mind when I read that question was COD4. I spent a loooooong time loving that game's multiplayer, I don't even know HOW long!
If I were to include MMOs,
Lineage II (would have) topped that list, considering I spent a good 3 years of my life sucked up in that game. I played other MMOs as well, but none as much as L2. Loved the ones I played more than a month (couldn't get into WOW, no matter how hard I tried, usually for the sake of friends who begged me to play it) and felt like, as far as games go, they were worthwhile endeavors. The reason my listing of L2 would necessitate past tense is because, like MMOs tend to, it kept getting "updated" as they year went by and... well, let's just say I hate what's become of it. =(
But yeah, all that is only considerable if I forget about Dota2. I played a LOT of
Diablo II, I played a LOT of L2, I played a LOT of COD4. But they don't hold a candle to the decade-and-going-strong I've spent playing Dota. XD
Good to reflect back on those other games, though. They were gems. =)
EDIT:
For me, you can't beat a bit of Halo Multiplayer (Not a popular choice around here, I know.)
Well, which one? 1? 2? 3? 4? The spin-offs? XD
I will say there's NOTHING quite like going to a friend's place with your friends, and someone brought their XBox and their copy of
Halo: Combat Evolved and hooked it up to the other XBox to be played with their copy of the game, and just have a full day LANing and laughing and killing each other virtually. When
Halo 2 movied to XBox LIVE, it lost that spark of friendliness and incentive to make outings with your friends for the good times. Sure, you could always meet up online, but that wasn't what made those outings such a delight! I sure do miss those. =|
