I figured questions and recommendations re: envelope filters pops up from time to time, and during COVID I got the opportunity to try a bunch out, and here's the results.
Note that I was looking for a classic Mu-Tron sound - nice quack, very touch responsive, can get a little bit stanky. but not TOO stanky so that it kills my speakers. I wasn't looking for a ton of flexibility and didn't want a ton of knobs, so the ones with the lots of knobs/options were out.
QTron (big silver box) - man, the tone on this thing killed. Just the right amount of stanky, very responsive - but the peaks were too much. Had I thought a bit more I would have figured out how to drop the volume when I turned it on, but as it is, it was just too tough to tame.
QTron+ - A little less peaky than the QTron, but otherwise very similar. Perhaps a little less stanky, a little less responsive than the QTron, but it was pretty close. Had the same issues with the peaks though.
MXR M82 - I like this one so much that I bought it twice. It's smooth - definitely didn't have as much of an issue with the peaks, it was very responsive, and very, very easy to get a good sound out of. However, it lacked the stankiness. I think in taming the peaks it lost a little something. Great pedal though, I used it as a grab-an-go filter for awhile because I knew that I could make it work no matter what the situation and I didn't have to worry about killing my speakers.
EHX - Microqtron - pretty much a small version of the regular Qtron+
Mr. Black Fwonkbeta - Stanky. Thick. Really Rich sounding. Still had issues with the peaks, but man, sounded great. Had a certain vintage vibe about it.
Source Audio C4 - Lots of flexibilities; lacks a touch of stanky, but does a really good MuTron. I still have this.
MuTron III - The sound I had in my head. Sooooo stanky. Had peak issues, but worth dealing with . I had to give this one back.
EMMA DiscumBOBulator - One of the best envelope filters I've played through. So easy to get a good sound, tamed the peaks, but tons of stank and rich, quacky tone. This one was literally a space issue. I was choosing between this and a 3Leaf Proton v3, and the Proton won due to space, but the EMMA is every bit as good.
3Leaf Audio Proton v3 - Everything about the EMMA is pretty much true with this one, except it is more flexible tone wise. It's an exceptional pedal. Sold it because ...
Micro MuTron IV - I got this with the "blue" opto module, and it's just phenomenal. Imagine taking a Mutron, taming the peaks, make it even more responsive, but somehow keep all the stankiness? With the blue module, the response is faster so you get a quicker peak, so it's like ... more. This is on one of my boards.
3Leaf Audio Proton v4 - This one is on my other board. It's the most musical envelope filter I've ever played. Very responsive, almost like it's an instrument in itself. It's very impressive. It's like they kept all the good parts and made them better. It's a little less stanky than the MuTron, but it's more musical, if that makes sense.
I also have a MuTon III clone coming, from a high end builder. I am excited to try that one!