Hmmm well it depends. All MMOs are addictive to those they draw in, not just WoW, though it is the most successful.
The more developed sandbox games like Oblivion and Saint's Row 2 are probably the runners up. I can't claim this of the entire genere because increasing numbers of Sandbox games are simply the abillity to committ some random mayhem between a story campaign, and maybe some races or something. The really good ones however have hundreds of things to do.
Right behind that would probably be fighting games, and FPSs as people will play hundreds and hundreds of rounds online and lose track of time... frequently. Both seem to be equal offenders in this, though FPS games are more popular at the moment.
Honorable mention probably goes to Strategy Games in general, however truthfully I don't think there have been any in recent years that have inspired the mania of say "Civilization", or "Master Of Orion" in their heydays. Recent versions of Civilization for example seem popular, but aren't really the "OMG" thing they used to be.
Hard to bring it down to specific cantidates however. If you omit MMOs I'd have to say the single most addicting game of late was "Fallout 3". I say this not simply because I like it, but because it managed to stay on Gamefaqs top 10 list for most of a year, oftentimes holding multiple spots for muliple platforms.