Ubuntu being the popular one, Linus operating systems are even getting more popular. Next to Ubuntu is Debian. I have  never herd of this OS but i am will to try it and compare it with Ubuntu.I would have guessed Fedora would have taken the second place. I already have Fedora running on my desktop and its fantastic.i believe Ubuntu and Fedora are the top OS systems but here is the list in order.Ubuntu, Debian, CentOS, Fedora, Red hat, Arch linux and suse.