Overall, probably Slime Isekai. It's got a good mix of wholesomeness, action, comedy, and world building. And it has just the right amount of OP in the MC.
Overlord is a good runner up. Great character development, good story and humor, intriguing world, etc.
Mushoku is easy top 5 but the bullying and horny drop it down a few notches for me.
Konosuba is also top 5. It nails the parody.
For non-anime, Reincarnated as a Sword is my favorite isekai light novel series, not that I've read all that many isekai light novels. However the anime adaptation was mediocre at best. It barely touches on the deep world creation and character development found in the LN.
For video game isekai, Shangri-La Frontier. No trapped in a video game nonsense. No completely unrealistic game mechanics. The game feels like it could be a real game. And the MC is OP only in the sense that they're good at games, not because of cheats.
Villainess isekai is practically it's own thing now and I have my favorites there as well but the classic is All Routes Lead to Doom.
I really love isekai so I could keep going but I'll stop here before I start talking about Trapped in a Dating Sim.