Gokusen is a Yakuza based anime(Japanese mafia). It's also a comedic school romance. Still one of the funniest animes I've seen, I really really liked it. If you can put up with the art style, you won't be disappointed.
Nisekoi is about a truce between a Japanese Yakuza and a American Mob gangs, brought about by a fake relationship imposed on the son and daughter of the gang leaders.
