First of all Linux does not represent all UNIX.infact GNU/Linux is a UNIX-Like Operating System.and to carry
UNIX trademark u have to comply ur OS with *www.opengroup.org standards and pay them for the license like apple did for OS X.
GNU/Linux is the best UNIX-like Operating System as of today because of its Open Source architecture+the popularity and the support for the hardware and other extra features.
I'd have selected FreeBSD;but the community is themself locked and the distro are not novice-friendly imho.
the worst UNIX ever made is darwin which is a messy codebase which is OS X of mac
OS X is sinking the good name of UNIX coz of the dirty hacks it uses to call it "idiot friendly"
Personally,I dont find Debian geeky and Ubuntu is just a small drop taken from the debian pool!I will always stick to Debian which is the best distro any time!
note that it requires a "used to *nix" user to switch to Debian.
FOSS nature of GNU/Linux is the basic cause of success.the organized way with a "lead" Linus who keeps the kernel fresh and neat and removes dirty hacks as much as possible.
One thing Linux kernel misses is a uniformed kernel API for developers to write drivers.this is due to different drivers which are needed to be corrected and devels are not interested at this moment I think.
Solaris(Open Solaris) is one good alternative UNIX which is growing under the lead of Ian Murdock(founder of Debian).We may expect a good OS with Open Solaris in some time(remember,Project Indiana),I hope.
Just read a week or so back that forefather of UNIX,Multics is released under FOSS license!wait and watch if some thing good turns out
and even read that multics is inherently superior to UNIX OS's.
*lippard.blogspot.com/2007/11/multics-source-code-released.html
*www.theregister.co.uk/2007/11/14/multics_source_code_released/
*en.wikipedia.org/wiki/Multics