Depends on what you want. If you are looking for easy install and mostly pre configured -->
1) Linux Mint : Stable with codecs and a suite of software installed for use by most users.
Interested in more of an adventure and more control over your system -->
2)Debian Squeeze is a great stable desktop with a large repository. Requires some more understanding of how linux works and you will be involved in the system set up from library and plug in install to additional software. But in the end it will be reflective more of how you want.
Interested in getting your hands dirty and dictating a large amount of the set up -->
3) Arch is my favorite since it is a very base install and requires the user to be knowledgeable of linux set up but gives you full control of options. The arch wiki is very extensive so there is allot of support if you need it.
I would also check out Distro Watch to "shop around" before you make a switch.