Is there a place that collects info on all these sorts of projects and allows you to compare specs and software? I bought a Retroid 2 on a whim but I'm wondering if it's the best out there.
https://www.youtube.com/c/ETAPRIME and https://www.youtube.com/c/ThePhawx are also good YouTube channels, although they tend to stick to higher end systems.
Finally, https://www.reddit.com/r/SBCGaming/ has a pretty active community. There are also a couple of good discord servers that I could track down if you're interested.
Retro Dodo does as well.
"The best" is very subjective unfortunately. And the answer will change based on what you're looking for.
IE I've searched for "the best" and ended up with an android phone in a telescopic controller before getting a Steam Deck.