Ubuntu is sometimes thought of as the beginner's distro, something that more advanced users don't care about - they think if you're advanced, you need Slackware or something where you have to build it from scratch, rewrite or add parts to the kernel to make it more compatible to your system, write your own drivers, etc, but that just takes up time. Of all the distros I've tried, Ubuntu is the fastest, most reliable, and (most of the time) most compatible.
I'm serious. Ubuntu is the fastest I've tried. Fedora and openSUSE are the other distros I've actually spent quality time with, and Ubuntu is a huge leap ahead Fedora. openSUSE just took a long time to boot up and still has a splash screen, which is dying out.
Now, I'm serious : Ubuntu is the distro that crashes the least with me.
Of course, the last reason isn't the best. Sometimes I can only find binaries in RPM format online, which I prefer over manual compiling because sometimes it can be such a pain to compile. But, let's face it: Ubuntu has a HUGE list of available packages for download and you can find most anything there. Fedora's pyrut visual package manager is slooow and takes a long time to browse. openSUSE's visual package manager is also slow, and doesn't provide for faster package name searching like Ubuntu, but it's more acceptable than pyrut.
Now, compatibility doesn't always have to be with software - Ubuntu has currently been the only distro to recognize my Nvidia graphics card and allow me to run desktop effects. openSUSE could barely recognize it, and trying to install the drivers broke my install. Fedora, let's not even go there. It told me I didn't have an advanced enough graphics card to play SuperTux!
Let's face it - Ubuntu has been a work of art in open source. I have yet to see a distro with such a punctual release schedule, team organization, and user resources.
Thanks to all the readers for making this my most-read post with over 11,496 views Tuesday 18 March 2008 and 22,843 Wednesday 19 March 2008, with about 1000 diggs and 70-something comments! So many visited the second day that the counter stopped working!