quickest possible and stable serial connx Arduino <=> RaspberryPi ?

Projects General Guidance
21

thanks,
so is that 1,000,000 baud rate the absolute upper limit to the mentioned 16MHz AVRs ( Nano, Uno or Mega2560),
and what about the absolute upper limits to the rest of the mentioned boards
Zero (M0, 48 MHz cpu) <=> Raspi,
Due (M3, 84 MHz cpu) <=> Raspi,
and esp8266 (80 MHz cpu) <=> Raspi
:?: