Win7...
I took the opportunity to go 64bit - something that I know I could have done with XP but back then there didn't seem to be a point - and to be honest I haven't been sure whether that alone is the reason for the speed increase I've seen.
Installation: astonishingly straightforward and fast, with the least number of questions to interrupt I can remember. Full credit too for getting them all out of the way early in the install. There are none of those annoying pauses where you come back to the 'puter to see it's waiting on you to click 'yes'. With 7, come back and it's all done.
Startup: much quicker than XP with a Vista-ishness to the appearance but none of the lag.
Interface: lots of nice improvements. I love the 'recently used list' has expanded to show recent documents for each application, and have been astonished by the breadth of driver and application support. I use a lot of esoteric apps and nothing I've asked it to do has created a problem.
Shutdown: miles faster than XP. Apps which stopped responding on XP just close properly and it's powered off in moments.
I'm quite cynical about Microsoft - the learning curve for Office 2007 for example - but I'm already evangelising Windows 7 to people because it is light-years ahead of XP and everything that Vista should have been.
If I have a criticism it is the marketing limitations that have artificially imposed restrictions to create different versions. I want to join my home domain and use the Media Centre. Why should this only be possible with the most expensive version? This use of feature-disabling to artificially create delineations should be outlawed. It leaves an uglyl scratch across an otherwise fantastic product.