To run Linux, FreeBSD, OpenBSD, NetBSD, or some other FOSS OS?
I’m running Fedora on a refurbished Thinkpad P1 Gen 4, and I’ve had good luck running Linux and the BSDs on higher end refurbished Dell Optiplex, Latitude, and Precision equipment.
Apple hardware is nice, and MacPorts gives me access to the vast majority of my *nix tools.
Shopping for new hardware I’d look at the list below to get Linux preinstalled.