Not counting obseleteness, I especially appreciate Windows 3.1, 2000 Professional, XP, and 7. 3.1 on anything 80208 to 486, 2000 on Pentium to Pentium III, XP on Pentium 4 or AMD Athlon, and 7 on anything else newer than Pentium 4. I think 10 has great hope though. Vista I really liked but paled compared to 7, same for 98 to 2K Pro and 8 to 10.
Linux I'm still avoiding so far, but I did enjoy a little stint with Archlinux on my netbook, more so than Ubuntu for example. It was ridiculously lightweight; if I had a less powerful netbook I'd definitely give it a fair try, but as it stands it runs Windows 7 Pro with ease.