|I use YouTube downloader (YTD):|
On the plus side this is one of the few that keeps right up to date when YouTube makes changes (quite often). It also allows you to select quality and size - flv and mp4.
On the minus side you have to untick all sorts of unwanted toolbars and other goodies during install - sadly all too frequent with downloads these days. It does remind you every so often that they have a Pro version but this is not particularly intrusive.
Always pop back and let us know the outcome - thanks