Hmm, good ask
@Danny , but I have a few questions.
1. Are we talking about underrated by the media, or underrated by Dolphins fans, or what?
2. Are we talking about from an absolute sense or only among the greatest players?
3. Do I only get to pick one?
All the suggestions so far are worthy of being the most underrated ... but there are others I would add.
For instance, if we're talking about underrated by the media, I'd go so far as to say the most underrated Dolphin was actually Dan Marino. He owned every meaningful passing record on the books during the era of a muuuuuuuuuuuuuuch more dificult passing ruleset, where there was no protection of the QB, and DB's could manhandle WR's anywhere on the field. His era required more accuracy, more bravery, quicker release, and better efficiency than today. Today the routes are short and QB's never fear getting blown up like they used to fear. The QB's do not need to throw the ball away almost ever. The personnel groupings are far more sophisticated today, making it a lot easier to find an open man, and to hit him. Big receivers can go unimpeded down the field and out-rebound a smaller DB for the ball ... and QB's get away with far less accuracy and far higher completion percentages. Then there's the bubble screen, which is simply a glorified handoff, to pad their stats. It's not really even worth comparing to the era of Marino. Yet, despite being clearly the best QB by far in his era, he had to contend with the preposterous notion that somehow Joe Montana and John Elway were on his level ... and they simply were not, even they knew it. They had better teams, better defenses, better running games, etc. IMHO, had Marino won 2 Super Bowls there wouldn't be a sportscaster alive today who would pick any other QB over Marino. But because he won zero ... he has been woefully underrated.
While Paul Warfield was an HoF'er, and the 1st pick in the entire draft, one could argue that he was underrated too. That team in that era simply didn't pass very often, and his stats look timid by today's pass-happy standards. But make no mistake about it, he was a Jerry Rice level technician and an Irving Fryar level athlete. Phenomenal player, who nobody even mentions any more.
But if you're asking who is the most underrated by the Dolphins fans ... it's hard to say, but I'm going to suggest Oronde Gadsden. He was a big clutch WR on a team that lacked options. The team struggled, but he always produced, especially in the clutch. Not a stellar athlete, he used size and physicality to snag balls. I'm probably the only person who would put him int this category ... and that's probably why I wanted to mention him.
If you were asking about the current team, I'd say it's Fitzmagic, who had a very decent season and has only ever been an afterthought by almost everybody as far as being a valuable member of the team. Ahh, such is the life of a rebuild .