Favorite lesser known anime

Kiratto Pri☆Chan is probably my favorite lesser known anime.

@Serafos I have never heard of that one. But it could be because I know nothing of Mahou Shoujo / Magical Girls besides Sailor Moon.

Arte
Isekai Izakaya: Koto Aitheria no Izakaya Nobu.
Itsudatte Bokura no Koi wa 10 cm Datta.
Kaidan Restaurant.
Kemurikusa (TV)
Kujira no Kora wa Sajou ni Utau.
Kuutei Dragons.
Ling Qi & Ling Qi 2
Makai Ouji: Devils and Realist.
Momokuri.
Neppuu Kairiku Bushi Road.
Ojisan No Lamp.
Rec.
Flame Of Recca.
Ryuu no Haisha.
Selector Infected WIXOSS
Senryuu Shoujo.
Shinrei Tantei Yakumo.
Shuangsheng Lingtan.
Somali to Mori no Kamisama.
The Tales Of Symphonia. Seasons 1-3 (OVA)
Uchi no Ko no Tame naraba, Ore wa Moshikashitara Maou mo Taoseru kamo Shirenai.
The Law Of Ueki
Un-Go
X (TV)
Yozakura Quartet: Hana no Uta.
Yuuki Yuuna wa Yuusha de Aru.
Zettai Karen Children: The Unlimited - Hyoubu Kyousuke.
Zhen Hun Jie

@Captain-577 It's an idol anime, but I guess it's not your cup of tea either.

@Captain-577 by your definition 99% of all anime isn't mainstream. So this is a pointless thread.

Hidden Gem Statistic:
Shoushimin Series
Atri: My Dear Moments
Blue Archive the Animation
Wu Shan Wu Xing
Yoru no Kurage wa Oyogenai

Potential other examples:
Wu Liuqi
Aki Sora
Mekakucity Actors
Undead Girl Murder Farce
Id:Invaded

The thing about "mainstream" is that, it can be judged in two ways based on where it's been discussed.

If it's in real life and you are in a region where anime is really popular, then Monogatari Series and SAO along with other titles that aren't as popular as the Big Three (Naruto, One Piece and Bleach) are still being heard of by tons of people in the community. Of course, if you are Japan...you know the answer to that already without me typing anything! xD But if it's like me in a tiny country in Europe dominated by drinking and sports culture, anime is a hit or miss. So majority will go for the Big Three, Death Note, Steins;Gate etc. For SAO and Monogatari Series, hardly anyone talk about it...well...anime in general unless someone brings it up in a conversation. Then again, I hardly meet any millennials or Gen Zs in my jobs. Mostly the workers are from Gen X who are most likely don't watch anime and more likely talk about the 60s-80s' bands or old western movies instead.

However, if you go online, not necessarily MAL, but any other anime related websites or forums, Monogatari Series and SAO were being talked about everywhere and both are definitely mainstream, if you stay away from the shounen mainstream anime topics.

Here are some titles below that I hardly (or never) see anyone talk about them, even when people recommending anime from certain genres. Some are my favourites while other ones are only average but still good:

Aria The Animation
Tamayura
Sketchbook Full Color's
Amanchu
Sola
Carnival Phantasm
Fushigi no Umi no Nadia
Nagato Yuki-chan no Shoushitsu
Yu-Gi-Oh! Zexal (and the eras after that)
Any Transformers anime series

These old titles were big in my home country in Asia back in the day, but not anywhere else:
Yadamon
Denkou Chou Tokkyuu Hikarian
Bakusou Kyoudai Let's & Go
Chuuka Ichiban!
Wanwan Sanjuushi (not the western version of Dogtanian)
Fushigi no Kuni no Alice (again, not the western version of Alice in Wonderland)

@LenRea Do you understand what "mainstream" means? Ofcourse 90% of anime isn't mainstream.

And that's how I describe it. As to what's "lesser known" is a bit subjective. (Still no One Piece, Naruto nonsense)

By the criteria used here for lesser known I guess the gundam franchise but I can't say it is really not that known in the anime community, a true lesser known anime I absolutely love is dead dead demon's dededede destruction

Soukyuu no Fafner: Dead Aggressor - Exodus
Yuusha-Ou GaoGaiGar Final
Yuusha Keisatsu J-Decker
Mazinkaiser
Giant Robo the Animation: Chikyuu ga Seishi Suru Hi
Change!! Getter Robo: Sekai Saigo no Hi
Choujuushin Gravion Zwei
Sentou Yousei Yukikaze
Jigoku Sensei Nube
Kaitou Saint Tail

First 10 that popped into my head. None of these are particularly obscure, and hell people that are into mecha and whatnot would probably be familiar with most of these if not all of them. But still, lesser known than most.

Death Note obviously and that anime that nobody knows what was it oh yeah Steins Frieren

fun fact Cowboy bebop is actually pretty much unknown outside of anime community

@chaosflame5 You are right about the subjectivity of "mainstream". And I am aware that Nisio Isin novels are absolutely mainstream in Japan (He is the author of Bakemonogatari series in case you didn't knew). But generally speaking, even people who know nothing of animanga will atleast know names like Dragon Ball, One Piece, etc. But I would put money on a bet that they don't know a novel/anime series called Bakemonogatari even exists. This is what I am going by. I discounted SAO because I remember even some non-otaku people had watched the first season and knew about it. But yeah, it's quite blurry.

And as for your list, I already have Aria and Nadia on my watchlist. I might try looking into others you mentioned.

And lastly, WHY THE HELL IS A YU-GI-OH SERIES ON THIS LIST??!

Death Note obviously and that anime that nobody knows what was it oh yeah Steins Frieren

fun fact Cowboy bebop is actually pretty much unknown outside of anime community

@Captain-577 I know about Nisio's novels on the Monogatari series, many people said the novels are pretty good.
If your post is about the non-anime people, then I agree, Monogatari Series won't be the first thing they will hear of. TVs tend to show Naruto, Pokémon and other stuff the most instead of Monogatari Series, which does have a lot of suggestive content from time to time.

Most of them are from the slice of life genre, while other ones are mild/mid action. I put Yu-Gi-Oh! Zexal and eras after that into it because the franchise got kinda died off around this point. Yes, real life tournaments are still around sometimes, but no one talk about the anime anymore except that one friend that I still meet from time to time for movies. Other than himself, no one around me talk about Yu-Gi-Oh at all. Nor anyone online these days.

I really like https://myanimelist.net/anime/42568/Yaku_nara_Mug_Cup_mo
The cast is great!