Gain and tone are big factors in getting an octaver to track well.
With the right input signal, my Line 6 M-series tracks very well.
It also has a mix knob so you can blend in as much of the unmodified signal as you want. This helps preserve your tone. I also has other variables (I forget which) so it's pretty versatile in getting the tone you want.
An added consideration is that the octaver is only one of about 100 effects builtin to the M-series ranging from various fuzzes and overdrives, to phasers and flangers, envelope filters, reverbs and echos, compressors, and it has a good looper too.
Spector club #243, Rickenbacker #487, Country Bassist #18