Since I'm now "going Monsterman" (so as to not have to register)
anyone have ideas on what muzzle brake to install?

I have two uppers that need new "muzzle devices".

Looking at:

CMMG

ALG Tactical (with canards)

Midwest Industries (Two chamber brake)

I don't have any experience with those but I can't say enough good things about my Surefire MB556K

I take it that you have a suppressor that screws on?

What is the intended use for this featureless AR?

Some comps are great brakes, but can be loud.  Also, are you going less than 16 inches?  I ask because I built up a 14.5 featureless middy and the brake and permanent installation adds to the cost.   Plus with a permanent install, better be 100% on the brake choice.

For range shooting and general "plinking" any brake will do.  The side chamber brakes are probably the best for this, but they are also loud.  As in they blast the noise to your side and onto the shooters next to you.  For these type of brakes, I like the Smith Enterprise "Good Iron".  

For general use and possible "home defense" I went with the Spike's Dynacomp on my featureless.  Its a good brake and not overly noisy.  A few other decent breaks in this category would be the Troy Medieval Muzzle Brake and Yankee Hill "Compensator".  

Some helpful reading:
https://sites.google.com/site/featurelessrifleguide/muzzle-devices

My "featureless" rifle will be my 18" barreled upper for shooting off the bench.

I figured that breaking the rifle open (as in BulletButton V2.0) would be a major
hassle when I plan to shoot 100-200 rounds "off the bench".

I also have a 16" Mid-length upper that I'd like to also shoot with that particular
lower.

(I went with the ALG Tactical for that upper, just to make it featureless)

Lantac Dragon

Been using a Precision Armament M4-72 on my 3gun rifle. Works very well.