The Boss OC-2 has been widely regarded as it. That's all, just "it", you see despite it not tracking the best and being approximately 30 years old there is something very nice about the tone ... especially on Bass!
A few years ago Boss then bought out the OC-3 which had an amazingly clever function, it could stop the octave effect at a note you chose. This meant you could have it running just on the lower two strings giveing a faux bass player effect.
The new OC-5 takes all of this, improves the tracking by a huge amount, adds an octave UP ^ but somehow simplifys upon the OC-3 design. Quite an amazing feat, this pedal will be appearing on boards across the world in the blink of an eye because everyone loves a new octaver.