Well it's definitely not the over hyped Ubuntu or any of it's clones and it annoys me to see so many recommendations for it from people who have basically used nothing else so can't possibly have a valid opinion on the matter.
I have used Linux for many years before Ubuntu was even conceived - yes Linux did exist before Ubuntu for all you bandwagon jumpers out there, and the other myth surrounding it i.e that it bought user friendliness to the Linux desktop is absolute tosh and an insult to those who have done a lot of work on other 'distros' before Ubuntu appeared
I've had a few favourites over the years, most of them sadly no longer with us.
Libranet was probably the first distro that had everything I wanted in a Linux system. Rock solid Debian with a friendly face and great configuration tools - The Ubuntu of it's time you could say, but better, much better IMO. On the founders death, his son lost interest and a great distro died with him
Then came Conectiva (sic) a Brazilian distro with great 'eye candy' and fabulous tools and one of the first to include all multimedia drivers as standard. It was bought out by Mandrake, which is why it's now called Mandriva and why it's so much better than the old Mandrake.
Then I moved to Slackware based distros, particularly Vector which has the right balance between Slackware solidity and lighter alternative desktops made to run lightning fast.
On my main machine I now use PClinuxOS (a Mandriva variant) and a mixture of Fedora Community Edition and Slackware variants like Vector or Salix on my lesser PC's
Vector is probably my current favourite, fast, stable and picks up new hardware effortlessly. I've got a hard drive with Vector installed and swapped it into lots of machines over the course of the last few months and it just reconfigures itself magically. Similar experiments with other distros are a bit hit and miss, so Vector is the one for me.
Honourable mentions to PClinuxOS, Mepis and the best Live CD ever - Puppy