Personally, it all depends on what you need from a Linux system.
Do you need a server or a desktop? Do you want the CDs to contain minimal programs to set up, or a whole set of CDs full of applications?
Ubuntu is my favorite Linux OS. It gives you just enough in the beginning to get you started, and there are different versions of Ubuntu depending on the window manager you prefer. Ubuntu has Gnome, Kubuntu has KDE, and Xubuntu has XFCE4. I'm not so fond of the new Ubuntu 6.06 install though; it's a GUI install during a Live CD session. I like the regular install, since it's quicker to get right into the install.
Some people have the wrong idea about Linux distros. Some people confuse window managers with Linux distros, since they are used to how other operating systems work. Basically, each Linux OS can do the same thing every other Linux OS can do. The difference is in the setup, the custom utilities, the repository software (aptitude, yum, portage, etc), precompiled binaries vs. compiling everything, and if you are looking for a server setup or a Desktop setup. Also, each Linux distro picks what services start up with the computer -- whether it be Samba to share files across computers or an ftpd to create an ftp server on your Linux box. Ultimately, once it is installed, you can choose what you want to start up with your computer and what you don't want. You may have a GUI to do so, or you may have to manually edit a configuration file. Either way, there is a method of doing this.
I'm quite happy with the repositories Ubuntu has. While I rather dislike the Gnome GUI and love the XFCE4 beta's GUI, I can find almost all the libraries I need in the repositories. For me, that's what I look for in a Linux OS. There is no ultimately best Linux OS. You must experiment and find what you like
