I would just recommend Linux. :-)
I currently have three PCs and all of them run with Linux (2 Fedoras and 1 Ubuntu) and in a couple of them there is also installed FreeBSD and OpenBSD.
By the way, I am a Unix geek-hacker (and, just to make things very clear, a hacker is NOT a cracker! :-) ) and spent 22 years of my life around the world developing and designing software, mainly, you guess, for Unix systems.