Win 7

I guess that depends on what you need...

Most people get by quite well with Win 7 Home Prem 64-bit, but Professional has nice features like remote desktop -- which most home users will have no need for.
 
home premium 64 bit is the best bang for your buck as long as you dont need/ want to address more than 16 gigs of ram, and dont need to be able to modify your network domain. also I have heard that some nas (network hard drives) have issues with home premium. so if your doing heavy nas, need more than 16 gigs of ram (system and video combined) or change domains than look upwards. (pro and ultimate cover up to 192 gigs ram)