Obviously the most popular OS is Windows as it's the most used... Billions of dollars in revenue can't be wrong...
I personally use Vista... but not by choice, simply an OEM OS and I really can't be fucked having an XP Partition or using Linux.
Besides.. Vista works fine for me, that is, other than the incredibly annoying WLAN Autoconfig which gives me a 4 second lag spike every 60 seconds. But I blame this stupid wireless connection instead of Vista.. bastard roommey needs to put the router in my room so I can use a Cat5e.. ya dig.
p.s. Some of you out there may have seen me say: "Linux is for poor people..." this of course is blatantly a joke on the basis that Linux is open source... but apparently a couple of you Linux nubs take it seriously and get all butt hurt... Just wanted to clarify that before you read my post here and call me a "noob."