Wow, some great answers here. A few comments. First, The rock Under appreciated??? The Fans (usually) love this guy so the belt on him every now and then was a good token, but it ain't neccesary for him to have gold to be over. Second, Yoko, this was a hell of a guy. For his World Title reign, he was built just right, Made to be evil, silent and unstoppable. He kicked out of a Hogan Leg Drop and then went on to crush Hulkamania. If you say he wasn't scary, then you weren't paying attention. If you're going to hate him ,say he was nothing, or call him a crappy champ, at least do it for the right reason, and that is that to counter him, Vince created Hulk Hogan Jr. (AKA All American Lex Luger, and his Lex Express) to fight him.
Angle and Jericho, both great wrestlers, talents on the mic, and overshadowed by thigs beyond their control. Hell Jericho was actually coming off as a great Heel Champ until the The Nose returned, and Jericho seemed lucky if he was being acknowledged as being in attendance at the Main Event of Wrestlemania (What with the Hunter/Steph show taking center stage).
Foley... not the greatest wrestler to hold the gold, but probably one of the most entertaining men ever in this sport, and definitely underappreciated for all he did (And I say thank God he got out before he had to be carried out on a slab) .
You want underappreciated, Chris Beniot (In WCW). Here's a guy who hardly ever got a tile reign, was stuck in long feuds with guys who had no business still in the ring (Dungeon of Dumb ring any bells) then, they say "Hey don't jump ship...have a title belt".
Warrior: I don't know about him being underappreciated. I was a fan of him at the time, and really if he'd have stuck around longer, maybe he'd have been more appreciated, but I thought for the time he was in the Fed he seemed to get a lot of appreciation.
One final name, a Man who I am pretty sure never won a WWF title belt, and may (or may not) have won gold elsewhere. A man who was a presence in the rig, and had a style unlike any other. A man who, as I said, never got any gold, YET probably should have (And arguably if not for personal Demons likely would have) Jake Roberts. Here was a guy who could do an interview, and without raising his voice, got across as evil, and plenty scary.
"Money's the Same whether you earn it or scam it" Bobby "The Brain" Heenan "Will you stop!" Gorilla Monsoon after just about everything Heenean says. "Welcome to Monday NYQUIL, where the Big boys Play with each other": Steve Austin, in ECW imitating his former boss Eric Bischoff.