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.
Anything over a decade old that isn't cash-cow shonen will already elude 50% of the casual audience. Casual consumers only annoy me because web searches for recommendations are worse than 15 years ago (SEO robot spam indirectly imitates what search engines think normies want).
Eden of the East, Monster, and season 1 Log Horizon come to mind.