@thumb I'm partial to OpenBSD.
FreeBSD is probably more popular both in terms of users and commercial reuse, but seems to be a bit more x86/Intel hyper focused? PC-BSD used to exist which kinda made FreeBSD a bit more GUI friendly from the get go, but I think whatever is left of that is probably now in TrueOS?
DragonFlyBSD seems to be the land of fascinating experiments which are too out there for mainstream FreeBSD.
Meanwhile, NetBSD jokingly will run on just about anything including a toaster. However, they do a lot of cross compilation (OpenBSD is very into making sure that builds work on their supported hardware first hand which is extremely useful) and their politics/fallout are why Theo went from being part of core to forking and creating OpenBSD decades ago.
There are other BSDs too, HardenedBSD is FreeBSD derived with more of a security focus, and MirBSD I guess was OpenBSD derived with a modus operandi I could never fully understand ("Mir" means "My" in German), being some examples which come to mind.
In general though, BSDs have sort of a distinct kernel /usr/src and ports /usr/ports but for me at least, still seem much more cohesive than any Linux distro and are much easier to work with source code rather than relying on package maintainers and such and dealing with binaries.
Having written as much, even OpenBSD now has tools such as syspatch and pkg to make working with binary patching easier.