Some seinen anime I recommend:
Planetes
Black Lagoon
Parasyte
Berserk
Hellsing (and Hellsing Ultimate)
Ghost in the Shell: SAC
Inuyashiki
Gangsta
Oshi no Ko
Golden Kamuy
Bungo Stray Dogs
Sing Yesterday For Me
Grand Blue
Basilisk
Yakuza Fiance
Kowloon Generic Romance
My Dress-Up Darling.