10. John Basil (Oz)
Though more than half of the "tits"-sniffing Oswald Penitentiary residents are eligible for a spot on this list, one man rises above the rest as Top User. Known more commonly as Jamaican inmate "Desmond Mobay," ambitious detective John Basil (Lance Reddick) assumes the alias when he goes undercover as a wannabe pusher in efforts to bust the jail's seemingly impenetrable drug ring.
Despite his surprising grasp on the accent, things don't go as easily for him as planned. To gain the trust of the prison's dealers, he's forced to endure multiple tests to prove that he's down with them, the first being to snort multiple lines of the product that he's been so insistent on buying, yet never seen using. Surrounded by spectators, he has no choice but to comply (even though he's been forbidden from using while on assignment).
The rest of the story isn't pretty. Clearly unstable in the wake of his first heroin experience, he's thrown into solitary, emerging with a raging addiction that's further fueled by other sadistic trials that his new friends put him through. Yet he's still adamant about forging ahead with the plan, doing whatever it takes get in with the gang—for his assignment, of course. Even if it means wiping out some members of his own team.