I think it's tough to beat the EH Microsynth for a single pedal, and with some tweaking, you can get some non-metal sounds. I say that as a lurker who's never posted here before, and has no qualms about disagreeing with anyone on this forum since I don't think I know anyone here.
The first thing to realize is that this pedal has a trigger, so you have to play very cleanly to restart the trigger, or take the opposite route and don't allow any pause between notes so you get the smooth synth sound. It also helps to roll the highs off, as the overtones can do funny things to the generator and you get strange occillating sounds.
The second thing that you might want to think about is how you can use the pedal for a whole slew of non-synth sounds, just unusual sounds that would take a whole lot of different effects chained together to create if you didn't have the microsynth.
There's a couple of examples on my band's website of the 'synth sound'. The "It's Getting Late" song the bass is all played through the Microsynth. On "Trippin' at WalMart," the Microsynth is on during the intro.(Well, most of the intro until we accidentally spliced in a preacher coming over radiowaves through the guitarist's not so well grounded amp....)
Check the songs out at our myspace site: Link Removed
It does take some tweaking to get some sounds, but as I type right now, I can't wait until the next practice so I can get in there and mess with that thing again.
The settings I used for the "Getting Late" song are something like this:
Trigger: 8
Sub-OCtave: 8
Guitar: 0
Octave: 5-ish - I can't remember off hand
Square Wave: 2 or 3
Attack Delay: 0
Resonance: 10
Start Freq: 8
Stop Freq: 4
Rate: 5
I think the key is not to get crazy with the Square Wave. While I do regularly use it, I only use it judiciously.
Another 'whompy' setting i use alot for the reggae songs is like this:
Trigger: 8
Sub-OCtave: 0
Guitar: 8
Octave: 4 or 5
Square Wave: 3
Attack Delay: 0
Resonance: 6 or 7
Start Freq: 8
Stop Freq: 3 or 4
Rate: 0
It's not very "synthy' of a sound, but it is fun, and I regularly use it live and in the studio.
You can also reverse the sweep (Start Freq: 4, Stop Freq: 8) for another good, not very synthy sound.