Uhmm. the most widely played online first person shooter in world, if Im not mistaken, is Counter-Strike with something like 6 billion minutes logged each month.
Being that CS is a free mod for HL, I wouldn't say HL multi-player sucks especially when you consider the other 15 or so other well known mods that offer both versus and co-op play.
Yeah, no idea how Halo multiplayer could even be considered in the same league as HL w/ Team Fortress, CS and DOD.
Half Life wins this competition by a landslide, though I'm judging from the PC version. I rented an Xbox with my bro and played through Halo in coop. That game got REALLY boring REALLY fast. The very last level when you have to drive that car out of the exploding base was really fun though.