Fedora
My distro of choice since F18. Longest streak without distro-hopping since Ubuntu run from 2007 to 2009. Fedora.Next seems like excellent initiative and F22 will be amazing with Wayland and KDE Plasma 5 defaults (Plasma in Fedora KDE Spin, obviously). Update: F22 only utilizes Wayland as default on GNOME login screen. While it has GNOME Wayland option, it is not yet default. GNOME is shaping up to be awesome in its own way as well with improved notifications and further refined applications. Update: In fact, both GNOME 3.16 and Plasma 5.3, which both shipped with F22, are so good that I have a hard time deciding which one to use as my daily desktop. Currently writing this small update from GNOME. 3.16 has fixed three of my biggest grievances: very slow animation speeds (no more need for impatience), tendency to stutter with many windows when entering Activities if dynamic desktops are enabled (now I can use them yay) and notification/tray area in the bottom. Window titles of default Adwaita on traditional applications seems to be a bit slimmer as well, so I don't have urge to quickhack metacity anymore. Not that I care much for GNOME nowadays, but still this is a excellent release. I have never been this excited about a single Linux distro version release before, and I've been using these things for a decade. In the end F22 really did deliver. This is just fantastic release.
Discovered an excellent post-install tool called Fedy. Just copypaste the install command on terminal, press enter, launch the application and you're good to go.
Think font rendering is not good enough? Check out Infinality.
RPMFusion is the essential extra repository for Fedora users, having packages that enable playback of pretty much any video format and other not-so-libre software.
Fedora COPR's are similar to Ubuntu PPA's, just easier to enable and centralized into one searchable place. Someone in IRC compared COPR's to Arch AUR, but that's not really the case in my opinion.
Use ksuperkey to launch KDE's search by simply pressing Super key. Get it here and read instructions. There appears to be a bug where it sometimes spawns another process when resuming from sleep. Just kill it. Totally worth that small nuisance. Update: unsure if it works on KDE5 anymore.
Korora is to Fedora what Linux Mint is to Ubuntu, providing codecs and extra repositories like RPMFusion out-of-box. They also explain why Fedora is a great choice better than Fedora project itself.
Did you know Fedora did away with the silly names? F20 "Heisenbug" will be the last named Fedora release. Those were fun. Especially "Beefy Miracle".
openSUSE
Old favourite of mine. I used it extensively from 2009 to 2011. Main strengths of openSUSE are technical, shipping with extremely good package manager zypper and YaST for configuring pretty much anything under the hood with a nice GUI (think Windows Control Panel). It also has the greatest KDE implementation of major distributions IMHO (arguably Chakra has the best one). But it isn't perfect. The default repositories are quite small so I had six extras setup at one point which really slowed down every package action, sometimes their security policies were absolutely retarded and so forth. But generally it's stable and robust.
Did you know Sailfish and Tizen make use of techonologies pioneered by openSUSE? They both utilize zypper!
Ubuntu
I can not really recommend this distribution or any of its many derivatives to anyone, especially not for new users. . There are just things that are done horribly wrong in this distribution. For the love of God, don't use it. Or if you insist, do not recommend it to newbies. Please read my rant about its inherent issues and their likely causes.
Some people like to complain that Canonical is hostile to good of greater community, since they often do their own thing with little regard to "bigger picture". I don't really think so. If they want to go their own way in matters of desktop, init system or whatever, it's their call and they are free to do so. Hell, for a short while Upstart was the technically best init system for Linux systems! And it was adopted by others! Let's say against all odds Mir really ends up being technically superior to Wayland. Then the initial powerstruggle between the two display managers will ultimately be a good thing. Their NIH Syndrome is their own choice.
Did you know Ubuntu logo is three people holding hands? It's pretty cool.