Boss OC2 (or thereabouts). I'm using a home-made OC2 clone at the moment, but have had great results with the Chord OC-50.
I've had the EBS OctaBass, EHX Multiplexer, Aguilar OctaMizer etc… and have come back to the classic OC2 sound; yes it's glitchy and doesn't track nearly as well as some of the newer octaves but it just [i]sounds[/i] great, and that's what it's all about.