Wally West
Fastest Man Alive
This is what the editor of Annihilation, Andy Schmidt, says about how Drax killed Thanos on another message board:
It's pretty simple, really. Drax has other powers that precent Thanos' natural Eternal-atom-control thing from working when he's around. Remember, Drax was created to kill Thanos. He probably couldn't beat Thor in a fight, heck, he'd have a tough time beating Wolverine perhaps, but Thanos he can kill.
Drax is essentially the right tool for the job. Heck, Superman couldn't kill Thanos--Galactus can't kill Thanos, can barely hurt him. But Drax CAN KILL him. That's what he was designed to do.
As for one panel, it took Drax the whole darn issue to get there. It took him several pages in the same room to get through the force field and such to get to Thanos. I personally think one great splash page is way more effective than 25 pages of the two of them beating each other up would have been. That's a matter of taste and we'll just disagree, I suppose.