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.
I don't watch much anime, but if you'll let me throw in a few manga:
ib-Instant Bullet. The original work of the guy behind Kaguya, and by his own admission his actual magnum opus that was unfortunately cancelled due to his inexperience leading to eh sales. Which means that unfortunately the final act is so rushed it gives you whiplash just to make sure he got the ending in.
But its a near masterful subversion of the "antihero turns good by the power of friendship and love" cliche you see so often. The protag opens the story by claiming that he is the villain, and he is then forced on a journey of love, friends, and all that gay shit. Only to find that it makes him hate the world more, because he finally has a taste for what he was denied for so long and is enraged that the world kept it from him. Its a very short read and the lore itself falls flat due to the aforementioned rushed finale, but I find that single "character arc" quite compelling. Unfortunately Kaguya shambled on for so long that his promise to revisit it is unlikely at this point.
Rosen Garten Saga. Unironically the funniest manga I've read in a long time. What starts out as a pretty bland ecchi comedy in the first few chapters quickly spirals into actual insanity as the rest of the cast fills out. The action is absurdly well done for what you'd expect to be a comedy and actually syncs incredibly well with the comedic and perverted aspects.
Its also just a fucking cornucopia of legendary heroes all represented in absurd ways. Alibaba is a bomb throwing lunatic on a motorcycle (because his "genie" was a smartphone that gave him advanced tech). King Arthur is a trap idol with enough strength to literally break reality. Beowulf uses a three sword style, but the third sword is his massive dong. Yamato Takeru is an actual madman whose brain only seems to understand murder and pleasing his father. Atilla the Hun is an ultra rich mastermind who is able to literally manipulate everyone at all times with setups going back 30+ chapters while also being a goofy idiot.
Probably not the kind of recommendations you'd want based on what you brought to the table, but if I can interest even one person I know they'd absolutely love Rosen Garten.