Can anyone suggest a good multi-tool that won't slip while using a torque wrench on the barrel nut or buffer tube nut?

Any tool can "slip" while installing a barrel or castle nut. That is not brand specific. Having said that I prefer the Guntec USA wrench.

The reason I ask is because I have read really bad reviews on all the multi-tools not being properly sized for the nuts or being over-coated causing the tool to slip.

Even with properly sized wrenches, they can still slip during installation. It all depends on angle, amount of torque applied, etc.

The PRI barrel nut wrench won't slip off .... I switched to this one after eating up a "multi tool" on a really stubborn barrel nut but the PRI does require that there is no GB installed so it can slip over the barrel.

ETA link fix

I'm looking into the Guntech wrench. My plan is to get a 6.8 barrel assembly from Bushmaster with the GB already installed and mount on a stripped upper.

I have had really good results with the DPMS Multi...

All barrel nut tools will have the propensity to move and slip regardless of which one you use and regardless of installing a standard barrel nut or a free float barrel nut.

Honestly the best method I have found is to always use a thick glove (like a leather or cloth garden glove available at any hardware store) and push the wrench into the barrel nut while torquing.  I use my left gloved hand to push the wrench into the barrel nut and my right hand to do the torquing.  The wrench has never slipped on me while doing it that way.