x2 on the original MXR envelope filter - it gives a surprisingly good Jerry-style EF effect, and it's dead simple to dial in. From the lack of controls you'd think it was an auto wah, but from the tone it's definitely more of an envelope filter sound - it offers up nice, round notes that vary with attack, and it tracks very well.
I have a MuTron III as well so I know what the real deal sounds like, and while the MXR doesn't have the range of a MuTron III, it's a very, very credible effect. I've spent some time over the last 18 months looking for something smaller/less valuable than the Mutron III that I'd be more comfortable hauling around. In that quest I've tried an unmodified Proton v1, a Proton v1 with the resistor mod, a Maxon AF9, a 3Leaf GR-2, and the current Proton with the decay knob. They each had their virtues, but frankly none sounded as good to my ear as the vintage MXR does, and it is BY FAR the least finicky EF I've tried. It kicked the latest Proton off my board, for whatever that's worth.
Dilettante - sounds nicer than "hack"