How are we defining "best" here? Power Level/Fighting Abilities, Plot Relevance/Impact, or Character Development/Personality? My "best" would depend on which of these is the criteria...
Power Level = Priscilla (Duh)
Coolest Ability = Riful (Runner up: The Destroyer only runner up since I'm not sure if she counts as two ABs or one)
Coolest Design = Isley (runner up: The Six Armed One)
Best AB based on the coolest fight scene = Rigardo
Most Developed AB personality = Riful
Most Dynamic AB Personality = Isley/Ophelia
Plot Relevance = Priscilla (Runner up: The Destroyer)
Most Warped AB personality from there time as a human: Priscilla (sorry Cassandra fans I just see her as releasing her inhibitions)
Most Evil AB: The Six Armed One (as in Evil requires being more than a simple hungry monster but going out of your way to commit evil)
Most human AB: Riful
You get the idea...