I use Ubuntu (Kubuntu, actually) but it doesn't really matter. I chose it because my server is also my desktop computer when I'm at school. I'd still choose it for a simple server purely because it's so easy. It takes me about an hour total to install the OS, all drivers, all additional packages, and tweak everything as I like it, by adding additional PHP packages, installing SSH and FTP servers, SQL etc. That said, if you have more complex needs, or you will want to re-install a lot, I'd check out Fedora and Kickstart.
From any Ubuntu installation:
sudo apt-get install apache2 php5 php5-gd php5-sqlite mysql ssh proftpd; sudo a2enmod userdir
And give it a few minutes. Enter the password once when the first command finishes. You've got yourself a testing server with PHP and MySQL, FTP and SSH, and you can just use your own user account by creating a public_html folder, adding your stuff, and seeing it online at
http://my.ip.add.ress/~username/.