Depends what you mean by synth.
If we're talking moog type sounds then I'd go with a combination of individual pedals. A cheap OC2 clone (or EHX octave multiplexer. I had a chord OC-50 which has a drive/distortion option which ALMOST negates the need for a fuzz), a cheap gated fuzz (an ebay woolly mammoth clone, or cheap muff clone, but the gated fuzz really helps) and an EHX micro or mini qtron (in that order) will be much better for these sort of sounds than any synth pedal I've found, and you'll get a good variety of sounds for little outlay, with the bonus you can use them individually.
For more spacey stuff the boss, behringer or digitech pedals would be better.
Or you could try the synths on one of the zoom pedals, B3, MS60B or B1on. They're ok...