hands down ...Delay

I feel delay is probably the most important pedal. you can add reverb-like ambiance that adds a richness to the sound, slapback for rockabilly and country type licks,soloing echos for a more rock-like sound,edge or brian may type repeats. as a pre delay (before distortion) you can get the Pat Travers type note doubling..if you can get overdrive from an amp and could only have one pedal.Delay would be the most essential basic need as far as I see it