The nice thing about XAMPP is that it is self-contained. It works well on all current versions of linux.
The generic answer is:
The best linux to use is the one you are most comfortable working with.
If you don't already have a preference, I would recommend Fedora Core because of the support community. A large number of people are very experienced with Fedora and are willing to answer questions. There are numerous tutorials available for Fedora.
I am using Fedora Core 6 and OpenSUSE 10.2 (server and desktop, respectively).
Note: because you asked for "the best linux," you will get a wide variety of opinions. I am not saying Fedora is the best linux. XAMPP will work as expected on all the linux versions I am aware of. Fedora is a simply a good place to start.
I am sure you will hear about Ubuntu, CentOS, Gentoo, and the BSD variants. One reason for the variations is that each version meets a particular need better.
Fedora Core tends to be the latest, cutting edge version. Nice for using new technology, but not necessarily for long-term enterprise stability.
CentOS, on the other hand, tends to be a year behind the latest technology, but it is very stable for enterprise use.
A lot of the web hosting companies that I talk with have been moving from Fedora to CentOS for long term stability.
I like OpenSUSE for the desktop because it makes it easier for Windows users to transition over to Linux. But I would not use it as a preferred linux server.
Todda, I, too, would like to know what linux versions other XAMPP users prefer and why.