Ubuntu is a very well put together OS for sure. It is based on Debian which is a solid, proven OS. The Ubuntu "community" as they call it, is rather large and there is a lot of input in its development.
There has been a lot of emphasis on usability, and many Windows users that actually try Ubuntu's brand of Linux like it because it is one of the easier Linux distros to learn for Windows regulars. So they say, and I am seeing that for myself as well.
I haven't used Mandriva, but I have used Fedora 10 and Puppy ( the latest, not sure of the version #). Still like Ubuntu the best personally speaking.