I tried several octave pedals (MXR, Boss, and one or two others) down Denmark Street a few years back and I found that the Aguilar Octamizer was the best; it had the best "feel" and kept the character of the original note (it must've sounded good, because other folks in the shop complimented me on my playing!).
However, it does struggle with tracking low notes and the tonal character of my Ric gives it a hard time.
I recently bought an EH Synth 9 from someone (which is a fun pedal) and they were also selling an EH POG; though I didn't buy that, I think it is the best octave pedal I have tried.