To all those who mentioned disintegration, he actually does get disintegrated in one of the comics, during the Civil War storyline. He's chasing down a guy named Nitro, whose power is that he can explode himself with the intensity of a nuclear bomb and then reassemble himself afterwards. Wolverine is standing directly next to him when it goes off, and his entire body is vaporized. And he still manages to grow his entire body back, brain and all, around his adamantium skeleton and finish Nitro off.
Take of that what you will.
Having said that, there's a few people that could kill Wolverine. Ghost Rider, if he used his Penance Stare on him (Wolverine has killed an uncountable number of people, and has inflicted so much pain on so many people, I can't imagine he would be able to survive). If Magneto teamed up with someone powerful (say, Dark Phoenix) could rip the skeleton right out of his body and then finish him off. And so on and so forth. There's many different possibilities to kill Wolverine, but the simple reason that he is loved and adored by so many people will keep him alive forever.
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Man, I feel like a nerd...