One-Punch Man

Saitama started heroing as a hobby. Then one day, after years of training, he lost all his hair, but became indestructible and gained the might to defeat any enemy in just one punch. After witnessing his power, a young cyborg named Genos anoints himself as Saitama's disciple in hopes of attaining the same strength and so-called "wisdom" as his master. They join the Hero Association in hopes of gaining rank, prestige, and notoriety (and money), but also meet an assortment of other eccentric characters, as they battle to defend justice and defeat evil. However, what Saitama truly wants is a challenge, or some kind of excitement, to beat his overpowering boredom.

Synopsis from Anime News Network