My recs, roughly in order of what I'm predicting that you'll like the best, based on what you said.
- Tengen Toppa Gurren Lagann - mechas, action, epicness, plot twists? Yeah, you got all of them, while you follow the story of a bunch of people raised underground, fighting their way up.
- Chainsaw Man - fast-paced action in a world full of demons. The protagonist merges with one, after living in shit since childhood, and that changes his life. Expect lots of humour, and the intro becoming an earworm.
- Kimetsu no Yaiba - historical-ish setting. The protagonist is a demon killer, who's sided by his demonised sister (yup). Sword fighting, great visuals, interesting and atypical story.
- Dr. Stone - humanity has been petrified for thousands of years, and a single genius boy is trying to discover why, how, and how to revert it. The science there is 90% accurate, 10% fantasy.
- Fullmetal Alchemist: Brotherhood (2009) - you watched the "original" FMA (2003), right? This one has slightly better worldbuilding and plot, and it's better grounded in the manga, although the homunculi are more relatable in the original.
- Noragami - the story of a stray god, a girl who can see spirits and gods, and the spirit of a dead boy.
- Mairimashita! Iruma-kun - protag gets sold to a demon by his parents. And the demon actually treats him better than they did. So he goes to school, alongside demons, while pretending to be one.
- Dungeon ni Deai wo Motomeru no wa Machigatteiru Darou ka - a boy who wants to be a hero, fighting in a dungeon full of monsters, in a world where the gods descended and granted their blessing to mortals of their liking. Which gods? Yes. Romantic comedy with epic vibes.
- Kill la Kill - watch this one if you watched TTGL and liked it. Heavy nudity, but the same "vibe" as TTGL. Hard to explain without giving you spoilers.
A few classics that I feel like they should be mentioned, but don't relate well to what you said that you liked:
- Sen to Chihiro no Kamikakushi - movie, not a series. About a girl who's spirited away to a world full of yokai, trying to save her piggyfied parents.
- Tonari no Totoro - also a movie, not a series, about children who move to a suburban neighbourhood full of magical creatures.
- Neon Genesis Evangelion - psychological drama pretending to be a mecha anime.
Some old style isekais, since you mentioned that you liked Overlord:
- Log Horizon - also "transported into a game", except that it happens with a fuckload of people at once. Most focus is how those people organise themselves in their former MMORPG, now real life, as well as how they handle the NPCs around them.
- Isekai Maou to Shoukan Shoujo no Dorei Majutsu - two girls in a fantasy world decide to summon someone, hoping to enslave him to help them out... only to get their magic deflected and end enslaved. The world is the same as the game world that the protagonist (the summoned guy) played.
- Re:Zero - protag gets transported to a fantasy world. He doesn't know how or why. His main ability is to go back in time, when he dies; so he experiences death over and over and over.
- Mushoku Tensei - the protag starts as completely scum, but he gets slightly less worse over time. Great worldbuilding and characterisation.
- Benriya Saitou-San, Isekai ni Iku - a handyman gets transported into a swords-and-magic world. Big focus on character relationship.
I don't recommend the anime, but check the manga if possible:
- Elfen Lied - a discussion on what makes a human "human", morals, and the likes. The anime adaptation is rather poor to be honest, but the manga is decent. Content warning: gore, sexual violence, a girl pissing herself right off the bat.
- Hoshi no Samidare - a great story, but poorly animated, about a bunch of people trying to save the world from a giant biscuit hammer that you only see if you know about it, that's going to hit Earth and destroy it. While subordinated to a girl who actually wants to destroy the world.
EDIT: GreenZanbato mentioned Tensei Shitara Slime Datta Ken, or "That Time that I Reincarnated as a Slime". It's also an isekai, roughly with the same vibe as Overlord; but the protagonist reincarnates as a slime, and he's far more self-aware than dummy Ainz. The novels are also great to read.
