What is the best Ethernet adapter for MacBook Pro m1 2021?

No such thing as a 2021 M1 MacBook Pro yet. The current model is the 2020 model.

Anyway, there are many around. "Best" is in the eye of the beholder. Apple sells this one: The Belkin USB-C to Gigabit Ethernet Adapter - Apple

Anker is also a highly recommended brand: https://www.amazon.com/Anker-Ethernet-Portable-1-Gigabit-Chromebook/dp/B00ZZ6NW5E

Most aftermarket are using the Realtek chip, and suffer from dropout at high utilization. There is a thread on this subject nearby.

This issue is said to be at least 'much improved', possibly even 'fixed' in 11.4.