When I decided my pedalboard was going to be sort of a "modular floor synth," I did a bit of searching for synth pedals... treating the sound from my pickups the way a synth would (pulse width modulation, waveshapers, filters, choruses, echoes) was one thing, but I wanted to change my string sound for a real oscillator, and I was certain there was a pedal out there that could get it done for me.
I had limited success. For some reason, at the time I was searching, pitch detectors in a pedal were more "real" than pitch detectors in a midi hex pickup; I did not look favorably on midi converters, and wanted the whole system to be a pedalboard, with no midi involved. Most pedals I tried weren't exactly winners, and the truly exceptional synth pedals I've used aren't best used as a raw oscillator at the front of a modular system; they are entities unto themselves.
I have come to terms with the fact that pitch detectors are pitch detectors; a midi pickup does exactly the same thing as the pitch detector on a pedal, except that it is actually modular, with full control over the note information, where the pitch is sent, what oscillators it controls, etcetera. I have some nice synth pedals, but my Roland GK-3B serves most of my synth needs.
Robot Factory Lo-Tech No-Fi Bot
Electro-Harmonix Bass MicroSynth
Akai SB1 Deep Impact
Digitech XP-1000 (jetpackmods)
Boss SYB-3 Bass Synth
MI Audio Pollyanna
Boss WP-20G Wave Processor
Digitech XP-300 Space Station