Vista, and I hate it. It's ok in as far as User Interface goes... initially, but under the hood it's all kinds of retarded. Mostly in that I have effectively about 500MB of RAM, because the OS eats the rest.
Only reason I'm not using W7 is that they're charging WAY too much for it, or at least for the Vista->7 upgrade (which should be thought of as a bug patch, not an upgrade).
/rant