The most popular would likely be PC-BSD according to http://distrowatch.com/
with OpenBSD lagging behind.
Unix was always an expensive OS used mainly on supercomputers (schools, government, etc). The Unix (BSD) community tends to be smaller than the Linux communities and usually lags behind in features as a result. I have used PC-BSD and OpenBSD and both worked just fine, but I find Linux easier and the Linux communities (especially Mint!!!) much more friendly and helpful.
HP Pavillion DV7 laptop, AMD Turion X2/64, 250 GB HD, 4GB ram, Atheros AR5001 wifi, Cisco WRT54GS router, Buffalo Linkstation Quad 4TB NAS w/Raid 5, Dual Booting: LM9-64bit/Gnome + LMDE. Life is good!