Shonen:
Hunter x Hunter (considering your avatar, you've probably read this one, but I'll recommend it in any case);
Fullmetal Alchemist (great classic, both anime versions are top-notch too);
Rurouni Kenshin (classic, a lot of japanese historic inspirations too);
Jojo's Bizarre Adventure (an 80's classic, it's still running so you'll have a lot of chapters to read. P.S.: it turns into a seinen later);
Hajime no Ippo (80's classic, sports manga about boxing).
Seinen:
Berserk;
Kingdom (if you like wars and epics, it's about the Warring States period of China);
Vinland Saga;
Vagabond;
Dorohedoro;
Monster;
Lone Wolf and Cub (70's super classic).
That's what came into my mind.