It's hard to pick someone for overrated because generally this board has popped most everyone's bubbles on that, but I will pick "pre-WWF" Curt Hennig. Everyone seems to want to paint him as one of the best in the world before WWF but I didn't see it watching the AWA set. I thought he was very good, and obviously capable of doing a lot more than he ever did in WWF but I didn't seem as a "top 10" in the world or even probably the U.S. at that time period. To me, Blackwell, Greg Gagne & pre-Vader Leon White were all bigger revelations" on that set than Hennig.
Memphis_Mark had some good names for underrated but Phil Hickerson stood out to me. If he is remembered in "mainstream" wrestling circles at all it is as the fat white guy pretending to be Japanese PY Chu Hi. Phil had a long career and was involved in much better stuff than that goofy character.