For me, it's the Analog Octave Up. I love the idea of it, a spikey, piercing & messy octave up. I love the circuits and theory. I've breadboarded every Green Ringer tweak I've found on DIYSB. Built them into other effects. Breadboarded the Ampeg Scrambler, Fox Tone Machine, Octavia, JC Maillet's Super Full Wave Octaver, Jawari and everything else I can try.
The problem I can't get around is usefulness only on single-notes. You have to pluck hard to get a decent tone, and inherent gating totally prevents gentle playing and subletly. Double-stops not only sound nasty, but the intermodulation also causes a huge volume jump and have a fast decay. I want them to be useable for something other than the Purple Haze solo, Stoner / Doom noodling or annoying others but they just aren't.