I've agonized over this for years. I've tried pedals in combination, vocoders, and stand-alone synth units, and I still don't know if I've found what I'm looking for.
Currently, I'm using the EHX Micro Synth, which is essentially a one-pedal version of the Boss Slow Gear, a filter pedal, an Octave pedal, and a Fuzz unit. Does it get any better than that in the realm of stomp boxes?
If I had my 'druthers, I'd have a Korg X911, but even then I'm not sure that I'd be satisfied.
Am Std. Strat (Lace Sensors Gold/Silver/Blue, GFS Delay Mod Board) -> Pignose Detonator > Ibanez TS7 (TS9 setting) > Ibanez TS7 (Hot setting) > Rocktron Vertigo > Danelectro Reel Echo > Vox Valvetronix AD50VT