I'll contribute something a bit different, because everyone loves something a bit different.
Prunes and Custard isn't a synth or a fuzz or a distortion or a chorus or phase or envelope but it is fun combining it with just about anything to get synthy discombobulated yet recognizable notes out of it. By itself it's pretty nifty too but kicking in other pedals it almost qualifies the OP brief.
Disclaimer, I never used it to cop specific cover version synth sounds.