Mick Foley is the only man to have gotten at least one good match out of every wrestler he's ever fueded with. Now, he's been fortunate to work with a lot of the all time greats, that much is true. But he's had great matches with Austin, The Rock, HBK, HHH, Taker, and then even after he retired, going on to legendary hardcore matches with Randy Orton and Edge, and yet you just don't hear people talking about how great a wrestler he was. You always just hear about HIAC and getting his ear torn off. Foley, IMO, solidified The Rock and Triple H as mainstays, at times when they had just broken out of the upper mid-card and were in need of consolidation as main eventers. And then at age 40, he went and did it again, helping the world realize the Main Event-potential of Randy Orton and Edge. The fact that he is lost in the shuffle when people talk about all the people Foley helped "make". That in my opinion is the very epitome of underrated.
The mere fact that a thread about Foley has been hijacked and become (yet another) HBK thread just proves how truly underrated Mick Foley is. He's the Rodney Dangerfield of professional wrestling.