The Devil is a Part-Timer
The best thing about this series is that's pretty much a reverse isekai. Usually it's just a normal guy getting hit by a truck then being sent to a fantasy world to face against a demon lord. Now imagine the demon lord being sent to our world and working as a part-timer.

Overlord
The thing I love about Overlord is that the main character is roleplaying as a demon lord who's seen as an absolute genius, but doesn't even realise until it takes 3 secs for him to realise that he's been taking over the world.

Re:Zero
Most isekai are rather fun and lighthearted, but not here. Re:Zero is pretty damn dark with the main character having the ability to reset time by dying. And of course has one of anime's absolute best girls.

That time I got reincarnated as a slime
I love that series has a much more slow and calmer pace then most other series and that it has an incredibly OP main character that can learn any ability/skill just by consuming things.