Wow, that's like the Ultimate Flame-bait (J/K)... Lets see...
Personally, I use Fedora on my systems. The reasons for it are several, but among them the most prominent is its balance between CLI and GUI tools. The Red Hat tools (system-config*) are very nice and, even though many people think they are fairly basic (and indeed they are, for a reason) they are very good to have a basic configuration and from which to expand by manually tweaking.
Another reason I like a LOT about Fedora is its focus on the latest stable software, even though it may not always work as intended or expected. It is certainly a distribution very easy to use, BUT not necessarily intended for new users, as it won't always do things for you and it some time feels like you should really know what you are doing as there is no way to tell if you are doing anything wrong or not (until it suddenly explodes in your face). Another reason I like Fedora for is its speed. Though some people may say the performance delta across recent Linux distros is negligible (and I'm not talking about source distros like Gentoo or LFS), for some reason Fedora (in my very subjective opinion) feels a bit snappier than others, particularly in GNOME, which for some obscure reason GTK redraw seems to be a bit slower than Qt's.
I don't like Ubuntu, though I reckon it is THE Linux distribution nowadays for new users. Particularly those who don't want a rather steep learning curve, and focus more on learning a new GUI (like it would be the case with Mac OS X, for instance). There are several reasons because I don't like Ubuntu for, but most prominently I have come to believe it boils down to its focus. The one reason I dislike it the most has to be the lack of a proper root account, sure it falls outside of its scope, and you can enable the account at any given point, but the system lacks the account on a default installation, giving away its focus. I particularly dislike doing administrative tasks with sudo.
Debian is very nice, I like it and all, but for some obscure reason I don't like .debs, I feel much more comfortable with RPMs. Maybe I've spent much longer time with them and I'm more familiar with it, the .deb format seems to be (IMVHO) a bit less thought through than RPM, also dpkg seems to be more difficult to use than plain rpm. However, it's got one of the best front ends for package management, apt, where as yum is slowly evolving, apt has been very robust for quite some years. For the rest, Debian is my second choice.