For the AR15, there are many armorer's wrenches on the market. I'm sure that some are more ergonomic and some that grip the various parts of the rifle better than others when loosening/tightening, etc.
I like that Avid but I'll issue a cautionary tale: I assembled a rifle with A2 front sight and fixed gas block. The "closed" castle/barrel nut wrench necessitated removal of the sight in order to slip the wrench over the barrel to tighten the nut.
Just something to consider if you're an iron sights kind of guy.
Barrel Nut Wrench for AR15/M16 Rifles allows the shooter to put the exact amount of pressure needed on the barrel nut while attaching barrel to the receiver.