openSUSE, says me.
openSUSE has become my Linux distro of the day. It seems to run very well out of the box on my ThinkPad R52. Although I had the best luck using the DVD form of distribution because it included the firmware for my Intel Pro 2915abg wireless card. The CDs will require you to install it yourself or let YAST provide the update. Either is fine as long as you have an ethernet connection.
I’m impressed with the refinement that Novel and the SuSE community has brought to the table with this latest version. KDE has been tweaked and massaged very nicely into a corporate desktop with everything at your fingertips and not a lot of extra junk to get in your way. But linux power users (no, I don’t fall into that category yet) will find familiar customizations just under the surface.
I did cheat a little and installed VMWare workstation with an XP machine available – just in case I got stuck and needed to be available for work. But so far, I’m rolling quite nicely.. I’ve even found that my battery life has actually improved.