Which BSD best suited to run as VMware guest?

Well vSphere 4.x only supports FreeBSD 6.x-8.x while vSphere 5.x also supports Mac OSX 10.6-10.7 - so one of those, anything else isn't supported.

From practical experience I can tell you all of the Big 3 BSDs (FreeBSD, OpenBSD, NetBSD) will run in VMWare, but as Chopper3 said, only FreeBSD and Apple's OS X (Mach/BSD Hybrid) are officially supported by VMWare.

If this is going to be a production system and support from the VMWare folks is important to you I would stick to a stripped-down version FreeBSD -- If you only start the services you need and zealously defend against feature creep it should work fine.