Notice I didn't mention him, as I agree he and Chavo were the only ones mentioned in your post who were truly underrated.Obviously the term is subject to opinion. HaI'm surprised Umaga didn't receive the credit he deserved while he wrestled. I remember people complaining when they read something that said Vince was going to push him to the moon. Watch some of his matches. The fans (mostly IWC) underrated him and vastly underappreciated him.
AS I stated in another thread, foreign superstars will always be underrated in wwe, the only hope they have to be big is to become top heels (and as you stated is ridiculous that sir Steven "william" regal never had a world title reign)Currently, the most underrated superstar is Yoshi Tatsu hands down. He gets a good pop for his matches, he has a unique style, yet the last time i saw him he was tagging with Mark Henry.
Over the course of his career, i would say that Steven Regal is pretty underrated. He was a great heel, always got a good reaction. I don;t think everyone is supposed to win the World Championship, but when guys like the Miz and Swagger get top title reigns, it seems a shame that Regal never made it up there. (i know that it may have been partly his fault re: drugs).