I'm a big advocate of Octaver, (Gated) Fuzz & Filter, for synth sounds, but my favourite dedicated synth pedal by far (if you can find one still) is the Chunk Systems Octavius Squeezer. The interface is admittedly somewhat patience testing(!!), but some of the old school synth sounds available out of this thing are outstanding, plus you can build your own patches. Alternatively, if you can get hold of the Chunk Brown Dog and 00Funk Mk II, they are the synthiest of Envelope Filter & Gated Fuzz around. Combined they sound amazing, very tweakable with fantastic tracking.