I've owned too many synth pedals over the years.
Ibanez SB7: Fun with guitar if fed into a gain source. Fierce lazer screeches aplenty.
Akai Deep Impact: Tracks decently with guitar up to the 7th fret on the high E string. I got one back when AMS had them on clearance and sold it at a loss because it was defective.
Boss GT-8: The intelligent ring mod and guitar synth effects were hours of fun, especially when the latter of them was fed into fuzz.
Tim Escobedo Uglyface: I've now owned three variants of this circuit and wish I'd kept the 4ms one with an expression photosensor. A nice, basic lo-fi atari-esque filter synth.
Digitech guitar and bass synth wahs: I preferred the bass version with guitar, as it had more useful modes. Still a great option for quirky synth bloops on the cheap. Also great in a feedback loop.
Electro~Harmonix XO Micro Synth: I'm not sure why I got rid of this pedal. Maybe it was the monophonic limitations. Either way, it was a nifty way of creating pseudo-Moog textures and cacophonous filtered octave fuzz thunder.
Line6 FM4: I sort of enjoyed the Growler mode for a minute, but it ultimately sounded too flat and plastic for my liking.
Subdecay Noise Box: Reminded me a great deal pf the Uglyface, but a bot more filtery overall. I found it to be more utile than the Harmonic Antagonizer
Sunsine Harmonic Decoder: A pocket-sized PLL seemed like a good idea ... until I plugged it in. The tracking was bogus, and there was no gating. I think mine was defective, though I've read similar problems have been rampant.
Digitech Dirty Robot: Kind of like a Synth Wah on steroids with a more refined Yah-Yah effect (featured on GNX units and the GSP1101) built in. Though the tracking can be a bit dicey, and it doesn't handle complex chords very well, this was my go-to synth for a number of months.
Electro~Harmonix Mel9: I actually received this pedal by mistake when I ordered a 720 Looper from a major retailer and decided to keep it. So mich retro prog/psych win in a single pedal. The tracking could be a bit better, but it still works well for drones amd slow passages. The only serious issue has been occasional system crashes when it powers up (there are multiple threads on various forums about this problem).
Sinister Analog Soul Provider: I was hoping for another Uglyface variant and ended up with a less tweakable PLL variant. The dual oscillators are impossible to tune. Total poop for doing anything musical for the most part, but great if you're looking to make atonal noise and enjoy random harmonizing.
Catalinbread Perseus fed into a Valeton Katfish Auto Wah: A magical combo that yields textures that fall somewhere between the Ibanez SB7 and EHX Micro Synth. The right post-gain source can unleash lazer beams.
Electro~Harmonix Synth9: My latest acquisition. Better tracking than the Mel9 and also has improved response to playing nuances compared to its predecessor. The Solo Synth mode is a bit noisy, and fingerstyle playing works better with many of its other modes. It cascades well into the Mel9, as many of their modes complement one another when layering. Though there is no formant-filter-type effect, and it doesn't distort particularly well, the Synth9 will likely replace my Dirty Robot in the near future. A polyphonic approximation of Micro Synth textures is one of its many highlights.
patiently biding my time for Synth 9 demos.