Octaves i own:
octabass is good and tracks really well
boss oc2 sounds pretty much unique - tracks reasonably
dod octoplus is cool and sounds cool - i've had mine run cleanishly down to a low d
ones i have tried/ tested:
MXR BOD is as good as the the octabass IMO - i bought the ebs after a head to head shootout. the ebs was around the ton, and the mxr was a bit more. I'm pretty sure i'll end up with the mxr too eventually!
the ashdown i thought was appalling - very disappointed
EHX octave multiplexer - quite liked this one, sounded good and tracked reasonably. similar tracking ability to OC2 IMO
all of this IME and IMO of course!