Ubuntu is good and so is SLED (or Suse 10.1). Another worthy contender is PCBSD
PC BSD. It's been getting a lot of attention at osnews.com lately (deservedly so I think).
PC BSD is novel in that it:
* Runs the full FreeBSD 6 kernel.
* Runs full KDE.
* Installs very easily indeed.
* Has what they call PBI software installers, which are much like MSI files under windows. With PBI's, you download an application as a single file (a .pbi file), and then double click on it. You then run through the application installation wizard. There's no software dependencies to worry about, and the PBI library is getting bigger all the time. It's good biscuits.
Cheers,
Andy.