There are plenty of publicly traded large security companies. They may not be the absolute best, but they're not going to screw you over.
I would feel pretty comfortable calling G4S, Securitas, GardaWorld, DynCorp, Brinks, Control Risks, etc.
Alternative, you call the whitest of white shoe law firms and retain them to find a good security firm for you.
But when you suddenly become a billionaire, what do you do? Ring up some private security company? You would need to trust them not to screw you over. It's kinda boils down to you not being able to trust anyone, not even governments that might consider you a criminal.