Botvinnik is the most underrated of all time, the fact that no one mentioned him on this list proves he is truly underrated.
Botvinnik is a good candidate for most underrated. He was at the top of the chess world for a lot of years. Another possibility, as I mentioned in another thread, is Efim Geller, who had plus scores against several world champions including Fischer, Botvinnik and Petrosian. Another is Reuben Fine, who had plus scores against Lasker, Botvinnik and Alekhine.
An intriguing possibility for most underrated is the relatively unheard of these days Isaac Boleslavsky. He came very close to a world championship match against Botvinnik, losing in a tie breaker a 1950 match to Bronstein to determine who would face Botvinnik the following year. Boleslavsky was a pre-eminent theoretician who pioneered the ...e5 push in the Sicilian, demonstrating that Black had adequate compensation for the hole at d5 and the weak pawn at d6. Ditto for the d6 pawn in several variations of the KID. His work enriched both defenses and helped usher in the modern world of those two openings.
Botvinnik is the most underrated of all time, the fact that no one mentioned him on this list proves he is truly underrated.