Depending upon the bullet to be used, N-133, N-135, H322, H335 are fav match powders. H322 got a nod for being one of the cleanest in a test using AR-15s some years back.
Whatever powder you use, keep in mind that the burning characteristics of all rifle powders will change as the pressures change. What you use for a near max charge with a 73 grain bullet will definitely burn differently in a mid weight load for a 55 grain bullet. This difference can translate into more or less carbon left in the gun.