Hekikai no Aoin
A guy meets an ageless witch who hunts mermaids and must help free her soul from centuries of hunting. Written by Kagesaki Yuna, who also wrote Karin, AKA Chibi Vampire, but Aion is much more serious from what little I have read of Karin, but I cannot be sure.
A very bland guy who is skilled at sewing is secretly dating the most dangerous punk on the planet while keeping it hidden from her all-female band of thugs, who has seemingly insurmountable strength. This is an absurdist romantic comedy in the same vein as Seto no Hanayome, and is similarly wonderful and dumb.
20th Century Boys
A group of childhood friends have to meet up after years apart, because someone they once knew is now a cult leader working for world domination and is basically the Japanese Antichrist. A live action movie trilogy was based on this series if you would rather watch it instead. Written by the same legend who wrote Billy Bat and Monster.
A guy is bequeathed a magic sword by the chosen one from 50 years before, and must embark on a journey to save the world from evil magic stones only he can stop with his magic sword and oddball associates. This was written by Mashima Hiro, who writes tons of schmaltz and stuff like Fairy Tale later on, but Rave Master was the best he ever made.
Kiben Gakuha, Yotsuya Senpai no Kaidan
A girl seeks help from the mysteriously perma-absent student Yotsuba to solve a mystery, and gets way more than she could have ever bargained for. Good stuff, and they do not make stuff like this much anymore, because any series with suspense nowadays either ends up as a death game series or emo garbage.