Mark Howe is criminally underrated by history.
For starters the Flyers of that era are underrated. They were first, second and second in the league in points from 1985-87, finishing second to the Oilers twice. They were stopped from winning the Cup by the Gretzky Oilers at their peak twice, taking them to Game 7 in one of those tries without their leading goal scorer, and by the death of their goaltender.
Howe's the best player on that team, by a pretty wide margin. This team is very obviously better than quite a number of SC winners, as few teams are good enough to get to the Finals twice in three years, and losing to the Oilers is nothing to be ashamed about. For a team of that quality to not have 2+ Hall of Famers is very unusual.
For instance, the Brad Park teams that lost in the Finals had Jean Ratelle, and a very very old John Bucyk, and an old Gary Cheevers. Plus a 998 point scorer in Rick Middleton. The Caps teams that won Presidents Trophies had an all-time great in Ovechkin and Nicklas Backstrom.
The Flyers only had Howe. Brian Propp is close to a Hall of Famer, and I might argue he should be in, and Rick Tocchet is close to a Hall of Famer, and maybe he'll get in one day, but he was very far from his peak in those years.
Howe led the league in plus-minus at plus-87, which is one of the highest marks of all time. The only players to record a better number are Gretzky, Larry Robinson and Bobby Orr. Obviously it's a flawed stat, but that's nice company to be in. And it doesn't even take into account how good he was on special teams, where he was the QB of a lethal power play and the bedrock of a PK that was near the top in the league for many years (along with Propp and Poulin).
Howe wasn't durable, and didn't have the insane longevity of Chelios. But Chelios, at his best, was no more than basically an equal of guys like Stevens, MacInnis, Leetch, maybe a half-step below those guys. And definitely a step below Bourque and Lidstrom.