I have a thing for envelope filters, (but that's healthy, right guys?!) so I've tried most of them and the best thing I've found so far is the "MeatSphere" from the nice fellas over on MusicPCB.com
I've been through the old big box Qtron+, but I didn't like the size and 24v power supply, so I moved to an old Ibanez AF-9, but the envelope didn't stay open long enough for my taste, so I built the MeatSphere and WOWZA(!) it can do absolutely everything (perhaps too much...).
Another board I've heard good things about is the Nautilus from madbean... that's next on my bench.
Here are some shots of my MeatSphere build:
As you can probably tell, there are no knobs on the top row... I ordered the wrong size.
I did perform the Color pot mod by tying two lugs together... it made a world of difference.
If you have a few hours and want a really good filter, I would give this a shot. I would say the parts are about $100 total.