I guess by "overrated" and "underrated", we are implying "by knowledgeable fans who respect the history of the game"...?
Maybe Phil Esposito is under-rated recently...? I feel like when I was a kid (late-80s, early-90s) his name came up a lot, and it was usually "Bobby Orr and Phil Esposito" as a tandem. But today, he seems to be overlooked a lot, and the fact that his scoring dominance was greater than Mario Lemieux's in his prime doesn't impress anyone. Now, I'm not saying Esposito was one of the truly elite greats, and I think it's well established that he wasn't a 'complete' player, etc., but when you're outscoring the opposition by the insane levels he was from 1968 to 1975, and winning Cups and MVPs, you deserve more respect.
I agree with the Niedermayer choice (above) as a bit over-rated by the hockey media, especially. He was a superb skater and a very impressive player, but late in his career the hockey media went a bit overboard trying to make him out to be the second-coming of Ray Bourque or Nick Lidstrom.
I feel like Paul Kariya might have gotten the Esposito treatment and been under-rated lately... however, I was happy to see him make the Hall of Fame. His mid- and late-career, esp. two seasons in Nashville, tend to be ignored.
Makarov is probably under-rated in North America, but that's sort of understandable.
Another Soviet whose name I never hear talked about anymore is Tretiak. If we'd asked people in about 1989 who the greatest goaltender of all time is, he might have won the poll.