Another vote for the MXR Bass Octave Deluxe. Tracks very well; I can go all the way down to my low E, though the sound there is so dark that I usually stop around open A. What I most like about it, though, is the tonal flexibility it has. Like others, it has a blend control, which lets you adjust for taste or take out the dry signal altogether for a somewhat synthy sound. But the "growl" and "girth" knobs, adding two distinct flavors of octave sound, are terrific. The former has more complexity to it, the latter a nice big roundness to it, and it's great to have the option, as sometimes one of them is really the best fit for the sound. I've found the latter is nice to use with an actual synth, for fattening up midrangey patches. It also has an adjustable midrange-boost button, with different frequencies to choose from. If you need a little more punch or cut in the tone, it's nice to switch that on, or leave it off for a smoother sound. Best part of all this is that it's quite easy to use, so the features don't become cumbersome. True bypass, analog.
It's worth nothing that any octave pedal will require you to adjust your playing style a little. They work best when you play tight & clean, and long sustained notes can be prone to glitching as notes decay or as strings age.
It's worth nothing that any octave pedal will require you to adjust your playing style a little. They work best when you play tight & clean, and long sustained notes can be prone to glitching as notes decay or as strings age.