NXP
The mimxrt port targets i.MX RT crossover MCUs — Teensy 4.0 / 4.1 and the NXP EVKs. Lots of RAM compared with a Pico. Docs: mimxrt quickref.
Why pick it
When a project is RAM-bound (buffers, displays, audio) and you still want MicroPython rather than a Linux board. Flash procedures follow the Teensy loader or the NXP EVK docs on the download page for each board.