Arch. I was already running Arch on my netbook, and it worked beautifully, so when I got a new computer the first thing I did was install Arch on it. Haven't looked back.
The Arch Wiki (and Beginner's Guide) make it really easy to set up your system, especially when combined with the list of commonly used/installed applications.
It takes a bit more time to set up, since you're building your system from the ground up with exactly the applications you want. Once you do, though, you know your system inside and out, and you can customize it exactly the way you want it.
If you have a 'do-it-yourself' attitude, you'll love Arch. If not, you may be annoyed by having to set up things that Ubuntu sets up during the installation (major things like the DE/WM, and minor things like setting up international fonts). Personally, I appreciate knowing exactly what and where everything is on my system and not having the added bloat, hence the switch.
@pspeter3 - how has been your experience with Elementary -
especially your transition from Ubuntu? Things like package management (any apt-get changes), multimonitor support, wifi/sound drivers, etc. ?
They are doing really cool (and actually usable stuff) with features like Switchboard+Plugs, Synapse, Pantheon, etc. so am really interested in switching.
Because elementary is just ubuntu under the hood. The interface is different and so are the default apps but all the drivers and package management works identically. It's been a great transition. The power of ubuntu with the design of Mac essentially.
