Mine favorite Widow is Windows XP. and i mostly use Widnows XP 2007. Coz it's low wait for hard drive and fast working.
Later XP was my favourite as as it was so user friendly as compared to that of 2003 Edition. But now Windows 7 took that place, it is so quick, customizable & friendly to use.
Without a doubt 7 takes it. I haven't gotten around to trying 8 yet but I'm curious to see whats new in it.
Windows XP used to be my favorite OS before Win7 came out. But after I obtained a copy of Win7 Professional, it later became my new personal favorite. I've tried other operating systems though, and I have to say that some of their features are also cool.