Linux for modifying?

Considering that the you can compile the kernel of your distribution as you want, no matter what it is, I think the most customizable distribution is Gentoo also for software. I say Gentoo because you have to compile every software you want to use since the installation phase. However Gentoo is not easy to use.

My friend has often suggested Debian to me as the most customizable Linux distribution. I have not expirimented with its abilities myself. I have used Ubuntu and Mandriva/Mandrake before and have been somewhat dissapointed with their customization level. I would say that of the two, Mandriva has the most customizable features.