By gem, I mean an anime that didn't exactly hit mainstream popularity but has a special place in your heart and you believe deserves more hype. That reason could be a rich storyline, well developed characters, an interesting concept etc.
For me, it would be One Outs. I'm a big fan of psychological and sport anime and I was pleasantly surprised. Despite being over 10 years old it is still one of my favourite animes that I come back to every now and then.
This show
Pop Team Epic
Awesome character development, very emotional story, & one of the best dialogues in anime.
Gankutsuou The Count of Monte Cristo
I just love this show. I tried it out of curiosity because I keep hearing about the Count of Monte Cristo book as a classic (I haven't read it though), and the similar name caught my attention. Although, the visuals are nice but kinda trippy and takes some getting used to.
Welcome to the Ballroom
Never thought I'd remotely enjoy a show about dancing, but here I am posting this at every thread like this one lol.
Land of the Lustrous
No pun intended
Of course you love kuma kuma kuma kuma
@Aphrody04 Of course I love Kuma Kuma Kuma, kuma!
People in this thread are really stretching the definition of "hidden" gem, huh? Like, I think Ping Pong is fantastic, and debatably the best anime of the 2010s, but I wouldn't consider it a hidden gem.
It's subjective, I mean a few of the suggestions I wouldn't consider to be a hidden gem but looking back on my own suggestion, it was the 6th most popular anime of 2008. On that basis alone it's not a hidden gem but the lack of discussion I see and my own personal preference for it would make it a hidden gem in my eyes.
Wolfs Rain
I'm actually due for a rewatch.. revolutionary girl utena is my favorite anime and it's not mainstream, but it still has a dedicated following. But wolf's rain doesn't have that (or at least none that I've seen), which is such a shame because it's one of my favorites. It has a great atmosphere and story (and a beautiful soundtrack) but it does have pitfalls.. and it's not the kind of show you can sit back watch with your brain off (i mean you can but then you're not gonna find it satisfying and you'll miss a lot).
Tiger Bunny
Tegami Bachi
These two are just the epitome of peak fiction