It really depends on what you want to use it for. Answers could be anything from a mini pc to an esp32

If you wanna go real crazy, take a look at these:

en.wikipedia.org/wiki/Cypress_PSoC

They have integrated hardware, so they are great for all sorts of electronics projects. A bit more towards electrical engineering than raspberry pi though.

Whatever old hardware you got lying around is pretty great. Especially old laptops because they’re usually power efficient.

And in terms of tutorials just plain old Linux stuff should work everywhere.

Used mini PCs, Chromeboxes, or laptops (the kind with removable batteries). As long as GPIO pins are not needed, most things can be replicated with minimal effort on Debian amd64. And when I do need GPIO, I can usually get away with an Arduino.