Hello,
I've been toying with BSD lately and it seems that XFCE is a good choice (not my preferred desktop environment but it works well on FreeBSD). And if you want to use a display manager, Slim works well too. For now, I have a FreeBSD on my X60t and it runs relatively well, considering it's stuffed with an old 5400 rpm hard drive.
I've also tried FreeBSD on one of my X220 and everything went fine but the laptop suddenly crashed as I was surfing using Firefox (and not the little crash, the laptop shat down without notice). I didn't reinstall FreeBSD on that machine but the latest Debian (9.6.0) and it runs flawlessly (I use LXDE or fluxbox, depending on the mood).
What I like with FreeBSD is the installation process, everything is well recognized, like the Wi-Fi (as I have lots of machines and as I'm running short on network cables, it's nice to install via Wi-Fi

). OpenBSD is nice too but not as nice as FreeBSD for a desktop OS. I haven't tried NetBSD because I wouldn't need that kind of OS. In the past, I've tried PCBSD (TrueOS nowadays) and it was not bad (with a graphic interface for the installation) but I witnessed some crashes too (on my cherished T60).
From what I read, the documentation of FreeBSD is one of a kind. I didn't need to have a look at it yet, but it's nice to know that you probably can find an answer to almost every question you should have on the OS :
https://www.freebsd.org/docs.html. It's important to me, because one of the thing that irritates me with Linux is the documentation (the man pages of Debian are well done though). You find tons of informations, but it's not necessarily for your distro and it may be inadequate. That can be rather annoying for a beginner (fortunately, I'm not a beginner anymore

).
W.