In terms of RTS games (real time strategy) the winner would have to be Starcraft. Starcraft is a game that can be easily picked up by almost anyone, but is very difficult to master. Perfect example of this, look at South Korea. Starcraft is nearly their national sport. Look up any finals match of any tournament and you'll find a civic center full of fans rooting on their favorite player. Starcraft has gone farther than any other game in the e-sports capital of the world and for this reason I think it definitely deserves the title of best multiplayer game.FPS games (first person shooters) don't have as clear a winner. If you go by sheer player count, Counter-Strike 1.6 is by far the leader, and for good reason. It is similar to starcraft in that it is an easy game to pickup but very hard to become good at. Skill level differences are magnified in both these games and I personally think that is why they have both been played on such a large scale for so long (10+ years). Any game that can stay active, let alone amass the largest number of active players of ANY game of it's kind, for that long certainly is doing something right.