Depending on the situation, I use a wide variety of Linux distros to accomplish the task at hand.
For my home laptop I use Ubuntu.
The reason I use Ubuntu is because it just works. It's very easy to maintain and it will interact with almost anything I throw it's way. I have been quite pleased with 9.04 thus far and have been using it exclusively for 3 months or more. It detected all my hardware and has been rock solid. I had to work through a few oddities like the flash player and various media codecs but that is to be expected. I don't have to think twice about this system, it just works.
For my home computer that acts as my router I use Slackware
Slackware is extremely solid. I will do exactly what you tell it to do. The problem is all the time you have to spend setting it up to do what you want it to do. Package management tools like apt-get and yum just don't seem to exist for Slack. You compile alot of things from source and resolve the dependencies yourself. However, once your done you can pretty much walk away from the thing and know that it will keep churning along.
I have a few CentOS boxes I use for various tasks.
CentOS is kind of in the middle between slack and ubuntu. Cent has the friendly tools for package management but you can also build things from source fairly easily. I consider it to be a happy medium between the two.
I have also dabbled with Red Hat before they started charging, mandrake linux, fedora, suse, puppy, knoppix, kubuntu, debian, caldera, and various others from time to time. Each one has it's good and bad points. I would suggest reviewing your options before selecting your distro. Depending on the task at hand, you have alot of tools to choose from
check out distro watch for more options http://distrowatch.com/