synthesizer as in guitar synth that need midi pickup or synth pedal that do multiple filtering type, multiple octave or fuzz like type?
For guitar synth that utilise midi pickup together with hardware sound engine, roland has their GR series. This type can get drum, piano, vocal choir, flute and many more others on guitar. This type doesnt care what guitar its being used on, as long as the string vibrate, the midi pickup sense this vibration, it will trigger the sound and send it to the amp.
For pedal type, different manufacturer have different type of synth effect for guitars. Some of it are fuzz like, some like octave, and some are filter type.
For the first type, selection can be limited
For the 2nd type, offering from digitech(bass synth), electro harmonix(micro synth, POG, HOG), moog(moogerfooger series are effect for guitar, but each of it are different part of a analog synth and can be used as table top synth instead of instrument effect) etc. You might like to google or youtube and check out some videos on the different sound possible
For synth with guitars, its much harder to recommend(as compared to any booster/gain type effect) so if really wanna recommendation, it might be good to search round online for the info/sound those equipment can make. And with synth, its never a case of plug and play device, it need the user to spend some time with it, get some good sound and find the right context to use it.
For flanger, offering from line 6, danelectro, electro harmonix, boss are decent sounding and reasonable priced.
