Picking the most underrated rapper in a city of 3,000,000-plus is a Sisyphean endeavor, but if you had only one ballot to cast, Leimert Park-based Co$$ would be a smart pick. Over the last 18 months, the close affiliate of Warner Bros.-signed Blu has quietly assassinated the few guest appearances he's been on since his sorely unsung last mixtape, "Tomorrow's Yesterday." And his latest, "Revelations," is an adamantine hour of head-nodding beats and face-slapping rhymes.