The majority of those are critically acclaimed manga,i wouldnt really call them underrated,more like not popular enough.Few of mine are:-
Lone wolf and Cub.(Infact all 3 Koike and Kojima manga.)
Vagabond
Berserk
JJBA
Real(one of the best sports manga)
Angel Densetsu
Law of Ueki(one of the best shonen ever written)
Sanctuary
Welcome to NHK
Eyeshield 21
Planetes
OPM
Mob Psycho
Me and the devil blues
Gintama
Oyasumi Punpun
Vinland Saga
TF
OMK
Kenshin & Renkin(Both are very good shonen)