What distro should I use?

The answer to this question depends on your level of expertise, the hardware that you run, and your own preferences.

The distros that I personally have used and do not dislike are Gentoo, Debian, and Slackware. Gentoo is extremely flexible and takes a lot of the pain out of building a customized system. However, such flexibility also makes it exceedingly easy for inexperienced users to shoot themselves in the foot. Gentoo is also not suitable for old machines that cannot compile code from source quickly. Slackware and Debian, being binary package distributions, are much more suited for older machines.

Slackware works well as a base system for admins that like to do everything themselves without the potential hassle of a package management system. However, it is obviously not well suited for server machines that require frequent maintenance.

Debian works very well in a low-maintenance role, but has the potential disadvantage of being extremely conservative in updating to newer software. This trait can be an advantage on stable servers, but it is often very frustrating for modern desktop machines. Additionally, Debian can quickly become unmaintainable if one does not require exclusively on the package system.

There are of course other distributions. I do not use them, so I shall not comment upon them. I currently run Gentoo on most of my machines, simply because I like its flexibility in excluding unnecessary packages or features.