After much thought and trying to stick with the intent of the OP, three drivers spring to mind, Gerhard Berger, Roberto Moreno and Michele Alboreto.
Berger and Alboreto languished in the Ferrari during their dismal reliability era, usually scoring points when they finished by but consistently retiring with tech' problems or crashing because the car wasn't as fast as they tried push it. Their timing with regard to driving for other teams didn't help either as they either didn't seem to make it into cars ready (or capable) to win championships or were past their peak if they did.
Roberto Moreno will get my vote for the most underated though because he languished in low rent F1 teams right through his peak years. He did well in other formulas before F1 and was a stunning driver to watch in US series but his appearances were patchy. His F1 career used him up and his potential was never exploited by a decent team when he was in his prime.