I started with Ubuntu when it was at it's dawn. Quickly migrated to RedHat and it's variants. I feel comfortable with RedHat based distros. But, can work on any other distros like Debian and it's variants, Slackware, SuSe, Mandriva without a hitch

Ubuntu: bar none.
Reasons: many, professional and personal
How long: Since 6.04

My vote for Suse, as i'm using that right now.

I have used many versions, from kernel 0.97 all the way up to where we are now. I currently have Suse, Fedora, Centos,Pardus, Sabayon, Ubuntu, Debian and a couple of others installed on my MacBook Pro running under virtual box.

Strangely enough the only one that I've had any problems with, is Ubuntu as the Guest additions don't deem to hook into the Video stuff properly.

Dave

Debian has been my favorite general purpose distro for 15+ years, but I use backtrack more often as it is slick, fast, and I am a fairly security-oriented guy.

I actually prefer FreeBSD for general use, typically. Just a matter of personal preference, I think there are some nice features in various Linux versions depending on one's purpose.

Ubuntu here, as it's what I started using. Although I've worked on Debian and CentOS servers a lot too, but for the desktop, definitely Ubuntu.

I really like and get on well with Unity too, although that did take a few weeks...

Slackware (since the begining) is my favorite distro, but i'm using Debian, Ubuntu, Suse, CentOS, Redhat(Fedora), Gentoo, Knoppix and a few others as well.