Forget all the mainstream crap that even the normies will rave about like it's the second coming. I want to know about the most obscure titles I've probably never heard of or are so old people don't even realise they exist but are actually all really good.
Three picks of mine that might have not made their way on peoples' radars.
https://www.anime-planet.com/anime/heavy-object
https://www.anime-planet.com/anime/jormungand
https://www.anime-planet.com/anime/tytania
I love serious sci-fi a lot or stuff that borders on it, bonus points if it's got good animation with proper battles, these three anime fit the bill for me. Jormungand is more of a military style anime but I often find these anime that fly under the radar often tend to be the better quality ones rather than the more heavily advertised mainstream titles and isekai trash.
Depends what you consider to be mainstream. Shin Sekai Yori is awesome, but might be mainstream enough? Depends if you mean mainstream among normies or mainstream among people who watch a lot of anime.
If you mean the latter, I enjoyed Tsukuyomi Moon Phase recently. The vast majority probably won't really like it, but I went in with sub zero expectations and the atmosphere it created was something I liked to the point where it's gotten me to read the manga, so that's gotta be worth something.
I would also recommend Shin Sekai Yori.