S - Never really a bad pick. If it's possible for them to be a bad pick then they're overtuned when they're a good pick.
A - Masters/Challenger viable champions that can be picked very often.
B - Alright picks, not necessarily viable in all games.
C - Viable into some matchups IF you really know them well. You probably shouldn't waste your time with these.
D - Champions that are probably best in top lane but are so bad that I can't list them any of the other tiers.
Unlisted - Unseen in high elo. Generally speaking, if there are not D2+ players that main these champions in top lane in a good region they will not make the list.
S: Maokai!, Akali, Rumble, Riven, Jarvan, Kled, Gnar, Renekton, Jayce, Shen, Pantheon
A: Talon, Gragas!, Cho'gath!, Kayn, Camille, Jax, Yasuo, Gangplank, Galio, Irelia, Trundle, Tryndamere, Swain, Kennen, Heimerdinger, Kayle, Mundo!, Sion!, Poppy!, Malphite!, Vladimir
B: Ekko, Fiora, Olaf, Lulu, Warwick!, Mordekaiser, Rengar, Darius, Fizz, Teemo, Wukong, Nasus, Urgot, Nautilus!, Yorick, Sejuani!, Rammus! Quinn, Lissandra, Singed
C: Ryze, Illaoi, Diana, Lee Sin, Shaco, Master Yi, Katarina, Karthus, Aatrox, Tahm Kench!, Soraka
D: Garen, Volibear
Champions with ! move up a lot when higher tier champions with ! are banned as they fulfill the same role.