What do you mean by "need a good GUI"? All of the BSDs you mention have XFree86 to provide a GUI environment, and they can run the same Window Managers that Linux does (KDE, Gnome, Blackbox/Fluxbox, IceWM, etc). That's a little like asking "which Linux has the best GUI?", it's not a valid question.
All three of the BSDs you mention would make a good workstation, what is really comes down to is which one you understand better and which one follows your priorities.
FreeBSD generally supports cutting-edge hardware features slightly before OpenBSD & NetBSD, it also has a lot more software packages maintained that you can add-on after the install. FreeBSD primarily concentrates on Intel and AMD architecture systems and hardware.
OpenBSD concentrates very highly on "correctness". The developers are so anal that they audit their code for misspelled words in comments & man pages, etc. They audit the code for readability, correct indentation & white space, etc... Even the man pages are much better thought out and more helpful than any other UNIX-like OS I've ever used (far better than the typical Linux man page). As a consequence of all the effort put into doing things correctly, there are less bugs in OpenBSD than most OSs and therefor less security flaws. OBSD also puts a ton of effort into making their system very difficult to hack into, but placing lots of road blocks in the way of buffer overflow attacks, and running services with the least privileges possible.
NetBSD is the father of OpenBSD and very similar in many ways, but the main goal of NetBSD is to run on as many systems as possible. No matter what type of really weird and obscure computer you have, NetBSD probably runs on it. They also have strict code audits similar to OpenBSD, but they're not as all-out paranoid about security the way the OBSD developers are.