I'd say Michaels and Flair are pretty much the top choices. One of Flair's main strengths in the ring was selling big time for his opponents, and making them look like a threat. Benoit maybe, too. Benoit would sometimes scare people into thinking he was really hurt (Wrestlemania 21, etc.) That's how good he was at selling. (Of course, maybe sometimes he really was in pain since he suffered a huge number of concussions that screwed up his brain, and we all know what happened there.) Bret Hart is another good choice, though I think his main strength was the ability to throw punches and kicks that looked the most realistic in terms of how hard they were hitting you, even though he's known to have never injured another wrestler (that would prove to be ironic, of course.)
There are so many potential choices for worst seller that I couldn't possibly narrow it down.