there is UNIX ,UNIX Like & UNIX Based Operationg systems
UNIX is closed source and you cant make use of it because it needs special hardware
UNIX Like is not fully open nor closed
UNIX Based is not fully open nor closed
FreeBSD is the best open source Unix Like OS
Ubuntu is the best open source UNIX Based (Linux) OS
Unix is not closed-source and does not require specialized hardware. I think the Open Solaris project is now dead, but I'm sure the source of the latest version is still available. There are even open source versions of Darwin, the basis of OS X. Unix is merely a set of specifications that an OS must have been certified to follow. I don't think there are any other Unixes, apart from OS X and Solaris, that run on x86 hardware.
In reality it matters little. All the OSes that you mention, along with those two true Unixes, are as close to "Unix" as matters for most of us. Which is better? Not a question that can be answered. Which is better, a Toyota pickup truck, a Honda Jazz, or a Ford Mustang? It all depends upon your requirements.