As far as I can figure, one can build an AR without any specialty tools but who want's to deal with that  aggravation. Since I bought two lowers, I'm committed to building at least two ARs so buying a few tools should be a good investment.  Here is my short list of "essential" specialty tools for a complete build.

 

Armorers multi tool.

Upper vice block

go nogo guages

 

What, if any, other specialty tools would you consider to be essential?

well for the lower you need at least the right size punch for the bolt catch release.  i wouldn't consider doing another lower without one of those pin retaining punches, a good punch, a good small brass hammer, and a allen key for the front takedown pin.  and a long screwdriver with different heads to get good leverage on the pistol grip.  the tango down i used shipped with hex head but my regular allen wrenches wouldn't let me get it tight enough.  a good screwdriver with an allen head and hefty handle fixed that.  i guess something to stake the endplate too

 

never built an upper.  

I would add a torque wrench for the barrel nut, but not required.

some tool for the castle nut too.  and a automatic center punch to stake the endplate for $10