Not underrated necessarily but not mentioned too often is Khaosai Galaxy. Dominant super flyweight champion from ‘84 to ‘91, 50-1 record, vicious puncher and probably the best out of Thailand. He’s definitely respected by Ring magazine and Boxrec.
Jeff Chandler. He has the 4th most wins in lineal BW world title fights ever. Only ever lost twice, one avenged, the other badly weight drained in his final fight. Beat Julian Solis x 2, Jorge Lujan, Gaby Canizales, Oscar Muniz and Eijiro Murata.
I was going to write another spiel about him earlier in here. Among all the experts and public at the time he was a star and always bracketed in with the likes of Hagler, Leonard, Holmes, Gomez, Hearns, Arguello, Spinks, Sanchez/Pedroza etc etc etc. He was considered top shelf with those guys. Sure he often came in near the bottom of the 10 for best fighter in boxing but have a look at the company he kept in that 10. I'd nominate Pedroza as well but this forum has been right onto him for over 10 years now and so were the writers by the time Sanchez died. If Blofeld isn't acutely aware of him being top shelf he'd be well worth the investigation. They came to respect him enough for quite a few to start thinking he could well come out on top of Sanchez in a unification bout.
If anything I think Galaxy is overrated, he gets highly praised and highly rated in all time P4P rankings. When he never unified and his opposition was very forgettable.
Well, I was going to say that he was actually crap and is rightly forgotten but I thought it might sit badly alongside my comment welcoming @Paul McB to the forum, you great plum.