While it is perhaps the seminal "trapped in a game" isekai, the pacing of this was so painfully slow. While the soundtrack is a masterpiece by Yuki Kajiura, it's used so awkwardly and repetitively that it doesn't do it justice. Skip this one.
The Devil is a Part-Timer!
Perhaps the original and best Reverse Isekai, this one will leave a stupid grin plastered on your face for its entirety. There's something immensely validating in watching epic fantasy heroes struggle to get by in the capitalist struggle for survival.
Avatar: The Last Airbender
Phenomenal worldbuilding, character stories and growth, complete with powerful and complex moral decisions. Plus genuine great comedy make this a must-see.
Soul Eater
Take Hogwarts, throw it into HalloweenTown, and add sentient weapons and general anime Shonen vibes. The result is a surprisingly enjoyable ride that tackles some heavy issues.