I've got an Aguilar Octamizer which I bought some years ago on Denmark Street; I tired several pedals, including MXR, Boss, and one or two others, but the Octamizer stood out because the octave below had the essential character of the note being played. The downsides of the pedal (and other synth/octave pedals) are that: (a) there's a point where it loses tracking and the sound breaks up, (b) they only go so far down on a bas; if you can get anything meaningful below A (or maybe C) then you're doing well. Aguilar are rather pricey kit.
I've also got a Mooer Tender Octaver, which "may be" a clone of a Nano POG; it has pretty good tracking and an octave up as well as down. The issue with it though is that the octave notes are generated and so sound artificial. It is a heck of a lot cheaper than the Aguilar, though.
In addition I have a Markbass Super Synth which has an octave function; I've tried it, gone yep, and switched back to the synth section.