Not really underrated for that nowadays. Not on here anyway, although I agree it used to be a common misconception.
I don't count TV or video. I've seen plenty of great fighters that way, but a lot fewer live in person. Probably the best on the night I saw him -- live, with my eyes and not through the lens of a camera -- was Tyson against Spinks. Fearsome. I witnessed Azumah Nelson probably around his prime, methodically knocking out Lupe Suarez -- same weekend, maybe even earlier in the day at Trump's before Tyson-Spinks. Saw Roy Jones Jr. against Thulane Malinga, a respectable foe, and my goodness what speed. Lightning. Oscar De la Hoya fought on the same card against a more hand-picked opponent -- great speed and skill but it was more like watching someone hit a heavy bag. I saw Lennox Lewis on the way up, against Tyrell Biggs, and Holyfield as a cruiser against Qawi the first time and later against Vaughn Bean and Bert Cooper. Holy against Qawi was still young and developing but what a fighting machine, what a will. Lewis was plenty impressive, but not really tested and still learning. Also saw Michael Nunn on one of those Don King starts-at-4-in-the-afternoon-ends-at-midnight marathon cards where Simon Brown, Oliver McCall and Simon Brown all fought against suspect opposition. Those are some of the more notables. I'd put Tyson that night over the rest at the time I saw them.
Salvador Sanchez. He had talent but he wasnt physically gifted in any one area other than maybe stamina. He was a complete well rounded champion that could adapt to any style and find a way to win. It was a privilage watching him fight. I was still pretty young at the time but even then I knew I was seeing something special. Anyway I dont think Ive seen a better fighter.