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

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.

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

https://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.

I own a few orange berry pi.

Nice boards.

A few OPi zero+ and an OPi 3b+.

Cheaper than rasps, quite similar hardware.

As everybody else says. Go with what you can find if you don't require gpio.

If you do require gpio and have a usb port handy, there is a module for you: Adafruit FT 232H

If you are not using the gpio you should look at mini PCs instead. More expandable and runs x86 processors, not to mention abundant on second hand markets.

Orange Pi http://www.orangepi.org/

A few advantages over Raspberry Pi come from its openness and flexibility. Orange Pi boards tend to be cheaper too, and they have a wider variety of models, including some built around RISC-V processors which are not locked to a proprietary hardware architecture. This makes it easier for to understand the system at a deeper level and to build truly open source computing projects.

Rockchip CPU based boards are much more price efficient and software is pretty good too. And Pi has the awful micro HDMI port

I like banana pi