Currently I upload on Sundays at 6pm (uk time). This seems to do fairly well for me at the moment. I'm not getting crazy amounts of views or anything, but, for the size of my channel I think it's decent. My latest video has 49 views and gained them fairly quickly.
I guess it depends on your audience, though, because of timezones and things like that I presume there isn't really a 'best time'. But, that's the beauty of YouTube, unless you remove/private the video, it'll always be there for someone to watch: immediately, a day later, a week later.. etc.