I think that Drapion should be in the B rank because of it's good attack, relatively fast speed, (it can outspeed many top UU threats such as Venusaur, Milotic, Moltres, and Mespirit and ties with Uxie, Houndoom and Arcanine the last two it can seriously threaten) and great natural bulk.
Poison may be one of the worst STABs available to a Pokemon, but Dark is an amazing STAB in today's metagame. It can run Swords Dance, a Choiced set, a viable Pursuit Trapping set, and Physically Defensive sets . It has only 1 weakness, absorbs Toxic Spikes, and is the fastest Toxic Spiker in UU. It's Special Defense and HP may be sub par, but it can still take a hit and retaliate with deadly results against the plethora of frail sweepers that have dominated UU recently. It has two particularly helpful abilities, if not situational, to take advantage of whether being defensive or offensive. Sniper, combined with the many high critical hit ratio moves that Drapion learns and a Scope Lens will send many Pokemon to their E rated deaths. Battle Armor allows Drapion to wall many Physical threats without the fear of death by hax.
Drapion may not seem to excel in just one aspect of of UU, but it's ability to do well in many seems to me that it should at least be a low B in versatility.