It depends upon the useage. On the server/downloading machine I run uberstudent linux for its lightweight default desktop and included programs.
For general usage, widest number of machines it will run on, and stability; Mepis 8.0 for machines with only 512MB of RAM, and Mepis 11.0 for machines with 1GB or more with the default KDE desktop. Also because Mepis is the easiest to configure for networking and file sharing.
For the Apple-users, I recommend Linux Mint with the Gnome or Cinnamon desktop, depending upon their preference.
On the main machines, I am currently running Mint with the KDE desktop for its better video file workabilities, and for future Virtual Machine configs.
On the older vintage 98SE machines I am running Pupppy Linux for its tiny footprint and ability to run on 64 Megs of memory and older CPUs.
Ahhhh! That explains why Mint annoyed me (other than puking within a week on the machine I installed it on).
I'm using Lubuntu v12s & v14s, because I like the desktop.