Zoom G1Xon. When I played in bands I had racks of stuff and a MIDI foot controller to run it all. This pedal replaces all of that and costs a fraction of the other gear, is a lot smaller, lighter and easier to set up. Does it do the job to my satisfaction? Sure does. Does it satisfy tone snobs? Nah. Do I care? Nah.

EDIT: Just re-read the rules and had missed the part about no multi-fx. In that situation my one pedal would be a volume pedal in the effects loop. This would allow me to control the amount of distortion which I would control by balancing the guitars volume against that of the volume pedal with the pre amp saturated. In a two pedal situation I'd add a delay, which can be used in many ways.