Jiralhanae Captain Ultra

The Captain Ultra is, according to Bungie.net, the highest rank a Brute can be. Though this is disputable, since there are no silver armored captains in the game (the bodyguards are the silver armored ones), and the Blue armored captains are actually the weakest of the Captains.

Also of note, is that the highest attainable Brute Captain rank as shown using the Thunderstorm skull is the gold armored Brute Captain Majors.