Subgenre of fantasy, where the protag goes to another world. Extremely popular, and extremely diverse, so this leads to a lot of less-than-inspired authors writing isekai. It’s really fun, though, at least in my opinion. [Disclaimer: I watch a lot of isekai.]

The Log Horizon series I recommended is IMO really good; a bunch of players of a game are trapped inside the game they were playing, and trying to come back to Earth. Other popular isekai series are:

• Overlord — it’s a single person reincarnated into the game. As the undead that used to be his player. It’s a mix of kingdom building and slowly watching someone’s morals fading away, as the habit makes the monk
• Tensei Shitara Slime Datta Ken* (aka That Time I Reincarnated as a Slime, aka TenSura; abbreviated names are common) — also “kingdom building” like the above, but there’s no game. Just some guy reincarnated as a slime. Mostly uplifting
• My Next Life as a Villainess: All Routes Lead to Doom! (aka HameFura, aka Bakarina) — the protagonist got reincarnated as the villainess of a game series she loved, and is trying to avoid the bad ending. Except she isn’t very smart.
• Re:Zero — the protag goes buy food late night, and suddenly another world, and he doesn’t know why. He has a weird “gimmick” though, he can return from death. The traumas pile up.
• Kono Subarashii Sekai ni Shukufuku o! (aka KonoSuba) — slapstick comedy. Protag kicks the bucket, and as he’s getting reincarnated the goddess can’t stop mocking him. He forces her to go to the other world with him. They build a dysfunctional adventurer party: he’s mediocre, she’s dumb, and they got a masochist and a mage who only knows a single wide-area spell in the party. [Note: not recommended as an introductory series for isekai, given it relies a lot on poking fun at common tropes of the subgenre.]
• Ascendance of a Bookworm — protag is a bookworm, dies crushed by books, and reincarnates in a world where books are extremely expensive and she’s dirty poor and has poor health, but she’s still obsessed with books at the expense of everything else.
• Saihate no Paladin (aka The Faraway Paladin) — protag dies as a shut-in, and gets abandoned when reincarnated as a baby. A ghost, a skeleton and a mummy raise him. Solid adventure, and rather good worldbuilding.