http://www.mangago.zone/read-manga/shamo/
http://www.mangago.zone/read-manga/freesia/
http://www.mangago.zone/read-manga/joou_heika_no_isekai_senryaku/
http://www.mangago.zone/read-manga/ajin/
http://www.mangago.zone/read-manga/dendrobates/
http://www.mangago.zone/read-manga/the_executed_sage_is_reincarnated_as_a_lich_and_starts_an_all_out_war/
http://www.mangago.zone/read-manga/akumetsu/
http://www.mangago.zone/read-manga/tomodachi_game/ (both underestimated and anti-hero)
http://www.mangago.zone/read-manga/it_s_mine/ (again, both of them)
second doesn't fit just as much with what you're looking for, but it's a good read on LINE Webtoon.
There's also:
http://www.mangago.zone/read-manga/darwins_game/
http://www.mangago.zone/read-manga/real_account_ii/ (the master of persoanlity switch)
http://www.mangago.zone/read-manga/fukushuu_kyoushitsu/
http://www.mangago.zone/read-manga/dorei_yuugi/
I’m looking for some recommendations with an under-dog plot. Kinda like an underestimated MC actually being strong or smart. Or an anti-hero one where the MC is morally vague and cynical. I’d prefer no romance but if it’s just a sub genre or isn’t mainly focused on the romance tag, I don’t really mind.