I suppose it would be the Arch live CD for me (technically a fully fledged distro.... but then I'm betting parted magic gets classed as that as well... it comes down to definitions of mini-distro ), as that's the one I use the most for anything that requires booting from cd/usb
Slitaz. When 3.0 comes there'll be Xorg, Openbox, all the default applications (including Firefox) in an iso under 35MB.