I use Debian (some reasons: excellent package system, including painless upgrades, and includes a zilion of packages, including all popular GUI environments, libs, etc.).

But bear in mind that my FPC setup is a little unusual:
- Many FPC versions "switchable" by symlinks, some of them stable like 2.0.2 and 2.0.4 and some regularly updated and recompiled from FPC SVN (trunk and fixes_2_0).
- Lazarus versions "switchable" by symlinks --- the stable one (compiled from tar.gz sources) and the one from SVN trunk.

I don't think that there really is a distro that is "better" or "worse" suited for FPC --- every distro has some lag between FPC releases and when it gets packaged for this distro. And FPC and Lazarus are trying to not be tied to any particular distribution, so they don't have any special needs.

In other words: just choose the Linux distribution that you consider "best" in general.