Of all the operating systems that I have worked with, Unix is my favorite. It does everything right. I love the multi threading, multi process architecture. Self Proclaimed Unix Guru here, By the way I love the raspberry pi, I am doing some rather cool things with this device right now.
Isn't it funny that many other operating systems are based on unix? It was also developed in the 70s. Me and a co-worker once, wrote some multithreaded racing applications, just to see if we could race the processes...Yes it was fun.
Systems Programming, Network Programming, large scale unix system architecture, yes... done it all.