First, let me define what I mean by perfect. One, the device is spec'd for 9mm rope; two, the descender does NOT impart twist in the rope. Three, it allows fast but secure rappels with very sure stopping power with 9mm EzBend and/or Maxwear. And four, optional, is a hands-off lock-off or stop.

Let's look at some possibilities:
1- the GG, Rig, I'D - none are spec'd for 9mm rope; imparts twist in the rope; does not provide secure rappelling with 9mm, may not stop a fast rappel.
2- the Cinch - not spec'd for 9mm rope, but close - 9.4 mm; does NOT impart twist; although it does not provide secure rappelling with 9mm you can add a ring for additional friction. Will stop a fast rappel WITH the ring
3- The Eddy - does not impart twist, spec'd for 9mm rope; does not provide secure rappelling on 9mm; may not stop a fast rappel.

4- Micro racks with hyper bars- does impart twist to the rope if the upper hyper bar is used. Rappelling control on 9mm is marginal.
5- the Scarab - not good for anything that I can see - imparts waaay too much twist to the rope; requires lots of wraps for controlled descent; friction not easily adjusted.
6- Figure 8s - not even in the ball park; imparts twist; low friction unless double wraps are used and then the twist is even worse
7- ATC type devices - very marginal for rappelling on 9mm, could easily get into a runaway; does not impart twist

Combinations:
1- the GG with an ATC or simple ring below it. Would provide a hands-off stop, but probably marginal performance. Difficult to keep the rope over the lip of the GG. The GG still imparts twist. The combo may be produce so much friction that it won't rappel effectively. Additional hardware (ATC of friction hitch) required. A short tether would be needed to attach the GG to provide room for the ATC below the GG.
2- the Cinch and ATC (or friction hitch) below it. Not bad, the rope does NOT have to run over a lip on the Cinch, so you would not have to worry with keeping the rope running over the lip like you would with the GG. Would provide a hands-off stop. The combo may be so much friction that it won't rappel effectively. Additional hardware (ATC) required and a short tether would be needed to provide room for the ATC below the Cinch.
3- a micro rack and an ATC or friction hitch below it - both the hitch and the rack would impart twist, but maybe only one would 'win'; I doubt they could be arranged to cancel each other; the rack would need to be tethered in order to get the hitch below it.

Sooo, I'm still looking.

You want a descender to impart twist??? There are plenty that do that!

Oops! typo!!! Thanks Bob, I inserted that little three letter word - NOT. What a difference that makes!

Yes, I am looking for the perfect descender, too. What I've had to settle for is: what combination of two items works the best, factoring in which rope I'm using, my weight, the type of tie-in, the type of hitch cord, the humidity, the amount of dirt on the rope, and of course, which side of bed I got out of in the morning.

The recall GG2 I got does not work well on anything I have now. It's either too slow or too fast. The original GG2 was pretty good on the fatter stuff. So I'm back to the Cinch until I can find something to slow up the GG.

LOL - it does seem to be that way doesn't it!

9mm is tiny, but I bet an OAR would work. You would probably need to make one with 4 or 5 rack arms and 1" rollers on a 3/8" spine. It does not twist the rope, and is easily installed mid-line.
If you could send me a short section of 9mm rope I could try to make one for you.

Gordon