I just got my hands on a Mission RJM expression pedal. It's incredibly responsive! The toe switch is perfect for a real Wah feel!
When the Wah is switched off it turns into a volume pedal
Just perfect
Some others pedals that I have tested are:
Korg EXP-2 use TRS-TRS cable. very good
Boss FV-500H use a TRS-TRS cable. In the exp input or seems more responsive set up with a "Y" insert cable like its a volume pedal
a little jumpy in calibration but doesn't seem to have a problem controlling effects
AMT EX-50 use a TRS-TRS cable. It dosn't seem to go all the way to the toe position in calibration, but just a fraction off. It's pretty cool for micro...
Ernie Ball VP jr. 6180 use the "Y" cable 2 TS (mono) to 1 TRS (stereo) seems good a little jumpy in the middle
If purching I'd recommend the Mission RJM or the Korg.
The others work and I had them already so I'm going to use them.
I don't know if I'd recommend purching them for this use? Inless you can find them used for a good price
hope this helps.