Taco Bell, until they got rid of the Mexican Pizza. Dead to me, now.
In-N-Out, if I'm living in a place that has them, which I'm not. Likewise, all of the cheap but amazing fast Mexican food joints in SoCal, most of which end in "-berto's" Alberto's, Humberto's, Roberto's. God I miss great, cheap Mexican (NOT 'tex-mex') food.
Of the remaining options, I guess I'd choose Arby's. Oh, and McDonald's on the very, very, very rare occasion I'd be in the market for a fast food breakfast.