Uh, that really depends on the use-case.

I like to follow the recommendations of the German PC magazine c't: https://www.heise.de/ratgeber/Bauvorschlag-fuer-einen-sparsamen-Heimserver-aus-c-t-3-2024-9587594.html

Other than that: An Intel NUC, one of those cheap chinese Mini-PCs from Amazon where you get 16GB of RAM, a fairly recent processor generation and 512GB SSD for like $250 or my advice: get a refurbished laptop for $250. That's energy efficient by design and has everything on board. And available in abundance.

Downsides of these approaches: You don't get a lot of SATA ports for harddisks, if at all... So for storage, I wouldn't consider those. So it's gonna be an old PC, Server or NAS. Comparing mainboards and energy efficiency isn't easy. That's why I rely on PC magazines. But that's for new stuff... Not used components. So tipps from the internet are probably your best bet.

If you're not from a country where electricity is that expensive, you might want to have a look at some of those refurbished PC shops. An server or a Dell Workstation from 5 years ago should be affordable.