When we look at high-scoring forwards, the "all-time underrated" players probably are:
Bill Cowley
Peter Stastny
Pat Lafontaine
Cowley didn't get going until he was 24 or so, and only contributed a lot to one Cup win in Boston, but still he had four 1st-team All Stars and 2 Hart trophies, and nobody but hockey historians seem to have heard of him. I don't think I've ever heard his name mentioned on Hockey Night in Canada, or on SportsCenter, or whatnot. I sometimes hear about Howie Morenz, or Eddie Shore, but I never hear anything about Cowley.
Peter Stastny has the 7th (or 8th, as McDavid is slightly ahead of him since a few weeks ago) highest PPG average of all time, and is the second-highest scoring NHL player of the 1980s and, indeed, from 1980-81 to 1992-93. For all of that, the only major NHL award he ever won was the Calder trophy, and despite seven 100+ point seasons in eight years, he never received a 1st or 2nd-team All Star, though it's a bit hard to see why he didn't for 1982-83. (As a reminder that PPG doesn't tell us the whole story, however, we should note that just behind Stastny is Kent Nilsson, who is also ahead of Guy Lafleur and Joe Sakic on the list!)
I feel like I'm beating the Pat Lafontaine drum lately, but holy hell he was talented! He outscored Mario Lemieux in the QMJHL in 1982-83 with 234 points in 70 games. (That's incredibly impressive. Yes, Lafontaine was draft-eligible a year before Mario, but he was born only 7.5 months before Lemieux, and was about three feet smaller!) The problem Lafontaine faced with getting his recognition in the NHL was that just as he really started hitting his prime (around his third season), the Islanders were starting to crap out. He started peaking just as the Isles hit rock bottom. Then at age 26, he went to Buffalo, a middling team but with high-end talent upfront, and then we saw what he could really do. In the 1991-92 and 1992-93 seasons combined, Patty scored 1.71 PPG -- second only to Mario, and well ahead of Gretzky, and Selanne in his rookie year. After that, concussions ended him, and he really only had one more full-ish season in 1995-96 (still scored 40 goals and 91 points despite missing six games). Lafontaine is in the top-20 PPG players of all time, and yet I'm seeing people putting Barrasso and Roenick ahead of him on American-player lists...? Really? Career-wise, I can see that, but in 'pounds-per-square-inch', if you will, it's Patty by a mile.
Looking at defencemen, I sometimes feel Larry Robinson might be a bit under-rated. Not that he isn't rated very high, because he is, but with six 1st/2nd-team All Stars, six Stanley Cups, multiple Norrises, a Conn Smythe, a really long prime, and the highest accumulated plus/minus of all time, I never hear his name in the "best D-man ever" talk. Sure, he's clearly behind Orr and two or three other guys (maybe more), but I think he's right up there. He's basically the prototype model defenceman. Chara is sort-of the modern version of him. I've seen people putting Brad Park over Robinson. I just don't see it.
Goalies? Don't know, I don't really care about goalies. Well, maybe Henrik Lundqvist. He kind of hit his stride around the time goalies were getting less dominant, and he doesn't have the single-season super flashy results to show off. But his sustained level of consistent excellence is pretty amazing, I think. Statistically, you can barely tell any one of his seasons from another between 2005 and 2016, and he had a great winning record, too, as well as two trips to the Finals in the playoffs.