(Young 'uns are exempt...for they aren’t fully developed which makes them suggestible to corruption or benevolence of the ''bigger boys''. 16 +, though still inclined to believe in the lies of the elders, are fair game/ )
To my mind...D'Angelo Barksdale, Lieutenant Daniels, Lester Freamon, Omar Little...
Because of its postmodern template and the disposal of grand narratives, each character has as much contradicting circuitry as the tangled narrative yarns....so this is a tough call. Most characters, at one point, have gone against their own moral inclinations and sin, sin, sinned. D'Angelo was a product of his environment, but had enough blunted intellect to realise he was involved in a toxic world. He often expressed remorse and craved some sort of redemption.
Daniels will stand against the hierarchy when his heart tells him to bluster.
Lester is the ''white'' version of McNulty.
Omar adheres to his own code of conduct, and only targets those who paint their own bull's-eye. He's not completely heroic, as Bunk opines, but he is one of the few characters who address their own personal faults.