I'm still fairly new to anime (~2.5 years) and have been just watching either classics/high rated anime or whatever everyone else is watching that season and really want to know what are some pretty good anime/manga that are underrated and overshadowed by huge influx of anime every season?
Mine picks would be:
One Outs : It's basically a Sports anime about Baseball with heavily focusing on mind games (similar to Death Note/Code Geass/etc) It is 8.4 on MAL but It's really underrated in terms of it's audience
Black Clover : It's really not that bad as people say it is. Yes the adaptation is bad but the story is really good, yeah it's basically generic shonen but it does the generic shonen things much better than other generic shonens. I'd still suggest the manga over the anime (with coming back to see the climax of arcs/seeing your favourite scenes being adapted)
Gokushufudou (The Way of the House Husband) : It's basically a manga about an ex-most wanted gangster now living as a house husband. It's one of the most funniest mangas I've read.
Vigilante: Boku no Hero Academia Illegals : It's a spinoff of My Hero Aca which is pretty cool but not really popular. The story could be boiled down to what would've happened if Midoriya wasn't able to pass the UA exams. Oh yeah and it's all canon (the main author verified this).
That's basically it for what I've read/watched.