Great: Amumu, Anivia, Blitz, Draven, Elise, Eve, EZ, Garen, Graves, Hecarim, Irelia, Jax, Jayce, Kat, Kayle, Khazix, Lee Sin, Lulu, Nami, Olaf, Orianna, Riven, Shen, Taric, Thresh, TF, Twitch, Vayne, Vi, Xin.
Good: Ahri, Akali, Alistar, Annie, Ashe, Brand, Cait, Cho, Darius, Diana, Janna, Jarvan, Kennen, Leona, Lux, Malzahar, MF, Morgana, Nautilus, Nocturne, Nunu, Panth, Renekton, Rengar, Rumble, Shaco, Sona, Talon, Teemo, Urgot, Varus, Wukong, Zed, Ziggs, Zilean, Zyra.
Average: Cass, Mundo, Fiora, Galio, Gangplank, Karthus, Kass, LeBlanc, Maokai, Mord, Nasus, Nidalee, Rammus, Ryze, Skarner, Soraka, Swain, Vlad, WW, Xerath.
Bad: Corki, Fiddle, Heimer, Karma, Kogmaw, Master Yi, Poppy, Sejuani, Singed, Sion, Sivir, Syndra, Tristana, Trynd, Veigar, Viktor, Voli.
Undecided: Gragas, Malphite, Shyvana, Trundle, Udyr, Yorick.
If I had to make a tier list, that's what it would look like. For the heroes in the "undecided" category, I just haven't seen enough games with them to make a proper decision. Or in the case of Malphite, I know he belong in either the "good" or "great" tier, but I'm concerned he's a little overrated.
By the way, a hero being in the "Average" or "Bad" category doesn't mean you can't own with them, they're just inferior to the top two categories. I consider "Great" to be the best heroes in the game, while "Good" is not as powerful, but still viable in the hands of a good player.