I'd place Ozai first. He was about as close to the definition of "evil" as you could get in The Last Airbender: cruel, ruthless, ambitious, and more than a little sadistic. I think he harbors more dimension than you give him credit for--he is where Zhao wanted to be. Zhao was also ruthless, cruel, and sadistic, but lower on the ladder--that blind ambition would have moved him up. Who knows? In a different world, he would have become Ozai's right hand man and the two would have conspired against each other (one to retain power, the other to usurp it). They really were the same sort of person, simply born into different positions. If the roles were reversed, I'm sure Ozai's overweening pride would have gotten him killed the way Zhao's did.
Azula is a close second. I'd rate her as being more bloodthirsty and vindictive than her father, but she's kind of the result of parental conditioning. Her basically evil father eagerly took her under his wing, while her mother shunned her. As we came to learn Azula acted like she didn't care, but when the chips were down, the difference in her parents' treatments mattered to her. It probably drove her further toward cruelty and domination, so as to retain her father's regard. Unlike Ozai (who was pretty close to inherently evil), Azula probably stood a small chance of being well-adjusted.
Zhao ranks third, for the reasons previously stated. I ranked him under Azula since she's probably more vicious than both Zhao and her father, though less naturally mean. Honestly, it'd probably be more honest to say Azula and Zhao are tied.
I'd rank Tarrlok just under Zhao. Tarrlok is manipulative, ruthless, and ambitious, but he doesn't carry that sadistic streak characteristic of Zhao, Azula, and Ozai. The guy just wanted to run shit and be popular: he wasn't afraid of leaving corpses behind in his pursuit of power, but would have preferred to keep allies and pawns.
Amon is simply fanatical. There's a ring of truth to his ideal, but it's tinged with madness. In his quest to make the world safe for non-benders, he's painted all benders as non-negotiable threats, which simply the case. I think of him less as evil and more as crazy/misguided.