Devil's Line. It's kind of edgy and over the top, but something about the romance in it is really compelling to me.
Also I'm apparently the only person who actually enjoyed Mayoiga as a horror/thriller and not a "so bad it's good watch". The ending's a little weak, but I loved watching it as it came out and trying to figure out what was going on.
There's a probability that the upcoming Anime Series I'll mention may not be underrated completely。Since they may have gotten the appreciation it deserved half way while it was airing。But from Romance to Super Heroes & let's not forget the Sports Series as well。Here are a few Anime that I believe to be under but enjoyed to the fullest。
✩K Project
✩Gugure! Kokkuri-san
✩Samurai Flamenco
✩Sakamichi no Apollon
✩Tsuritama
✩Space Dandy
✩Mushi-shi
✩Sangatsu no Lion
✩Barakamon
✩Nana
✩One Outs
I liked Usagi drop as well as someone mentioned above,
I would say Kare Kano was highly underrated but it's because they never finished it and I hated that! (I read the manga tho, so I got to know the end).
Honey x Clover is fantastic and I never heard too much hype about it.