I have been using Windows since 3.1 So I have watched the OS progress through its more awkward and clumsy manifestations into the refined and hallowed XP Pro, then a few steps forward and a few back with Vista. I hope sincerely (but I have my doubts) that 7 will someday finally match the reliablilty and ease of use that XP had for the layperson. The fact that a large percentage of the world is using a 10 year old OS is a travesty IMO. There is a lot to be said for Windows 7. The fact that I installed the beta and NEVER booted back into Vista afterwards speaks volumes. I never had any significant issues, hangups, crashes. or bugs (save a few insignificant driver issues) with the beta or the RC. The fact that I have used the pre RTM release as my main OS this entire time without any problems says a lot about MS's latest offering. And since installing the RTM I feel like this OS and I were meant for each other lol.
So to answer the question. I would say my favorite OS is Windows 7. I still love XP for its nostalgic interface, universal appeal, and stability. But truth be told it is dated, has many problems of its own, and im just all around sick of it. As far as Linux, I like Ubuntu and a few of the other distros but i find they never seem to work right out of the box without a bit of tweaking and configuration, which i don't mind but I think is still too complicated for the average user who just wants to install the OS and be done. As far as Mac... don't get me started. I'm sick of Apple and mac fanboys touting their expensive hardware and OS as the end all and always better than a PC. Macs are good with multimedia tasks and I'll give em that but as far as an everyday use multi-tasking OS, I'd rather have a Windows system over a Mac any day.