Everyone will have their preference, but most (myself included) regard the MKII as the best iteration. I have had all the different versions and liked the original MK3 where all the printing disappeared a few gigs in to the MK3.33 I got from a recent drop. I didn't have them at the same time so wasn't able to go head-to-head but I wasn't able to easily dial in the "3Leaf Octave" thing that I had got from the prior three iterations from the 3.33 so I moved it along.
Also FWIW, I am not currently using an octave pedal in my rig. When a tune calls for that sound these days, I play either a Moog synth or Crumar CPB-2. That said, if I were to go the pedal route for that sound again, ANY of the 3Leaf get you there better than most of the other octavers I have tried. The MKII sounded the best, but I know the form factor is an issue for some with limited space, but any of Spencers octavers will get the job done better than the rest. They track really well and have the versatility to get a sound between the OC-2 or Mu-tron.