I started on RedHat, then moved to CentOS when RedHat became none-free.
Then moved to Debian when CentOS was unable to provide an in-place upgrade from v.4 to v.5.
Now I'm gradually moving to Ubuntu LTS, for no reason other than its longevity. Most webservers now run 12.04 which is upgradeable to 14.04. I'll wait a couple years 'til all the bugs are worked out of that process (and 14.04)
Well, i learnt on Centos back in the day... But i have nothing but Debian and Ubuntu based Servers where i can.. (except for my Nodes are all running Centos)
But everything Personal is Running Ubuntu or Debian
You shouldn't base your choice of OS on a popularity contest. If you are new to *nix, and are looking for a place to start, going with the most popular is typically a good choice. Otherwise, stick with the distro that you personally prefer, or are most comfortable with. Intimacy breeds proficiency.
I have been using Ubuntu Gnome Edition for quite a while on my personal laptop/desktop as it "just works". For Servers, Debian and centOS are my distros of choice.
Centos & CloudLinux are my favourite, however for personal use it would be LinuxMint simply because Ubuntu which I used to love keeps trying to copy Windows UI and it's awful.
I started with SuSE, 'back in the day', LONG before they were bought by Novell
But anyway, my present favourites are openSUSE, Debian (and a few of it's derivatives - such as Mint Debian, as a desktop system).
Gentoo is another favourite, and, again - for desktops - Arch Linux is growing on me, recently I've grown quite found of 'Manjaro'. I also use CentOS on a few servers.