I you’re looking for anything besides octaves, the only real options are the Digitech Whammy V(the newest version with the classic mode) and the Boss PS6. I would get the Whammy(for just octaves or for other intervals) because I think that it sounds absolutely killer, but if you need really intelligent shifting (i.e. will play in key with you, unlike the Whammy where you’ll have to move the controller back and forth between major and minor thirds and actually know sufficient theory. The Digitech Harmony Man is an option but honestly the Whammy seems to be the only pedal that they try to make of good quality.
I don’t like the sound of the Boss shifters nearly as much as the Whammy, but if intelligent shifting in requirement then there’s probably nothing better than the Boss save the Eventide Pitchfactor.