3. Mitchell Tanner (Warhawk)
Warhawk is a highly trained assassin, a soldier, a marksman and an excellent armed and unarmed combatant. He's is vastly experienced with explosives, electronics and computers and possesses enhanced strength, enhanced endurance and steel hard "omnium" skin which renders him completely bulletproof. His weapon of choice is a standard high-powered rifle, but he has also used a gun firing tranquilliser darts and a flechette pistol which fired rocket-powered darts. He had a stint in the CIA, during which time he was hired to kill various supervillains, and he fought in Vietnam (which would obviously need to be updated to a more modern war if he was to appear in Agents of S.H.I.E.L.D) and it rendered him insane - causing him to do crazy things like attacking superheroes. The CIA released him as a result and he became a free agent and was recruited by S.H.I.E.L.D. They promised to reduce his punishment for attacking superheroes if he served them well. A very interesting potential plot.