Diamond Dallas Paige
the diamond cutter move was done almost any way possible most versatile and hype at the time
Selling means making your opponent look good. It's not the act of a guy doing his finisher.
For example, when Rock had a Stone Cold Stunner done to him, he would flip backwards in ridiculous fashion. That's how he "sold" the move.
Anyway, on topic - a lot of people are saying Ziggler. Ziggler is great, but he's a little over the top with his selling. If I'm saying all time, I'd have to go with Flair. Flair made his opponents look like fucking GOLD. I don't think people understand how many stars Flair made simply by selling for them. Sting would not have been the legend he is considered to be today without Flair making him look like an absolute monster each and every match.
If I'm picking current guys who sell like champs, I'd go with Rollins, RVD, Slater and Ambrose, just to name a few. HBK was another all time great at selling (aside from that joke of a match against Hogan), but he's no longer an active wrestler.
Honorable mention:
And...just for good measure...