Nidalee is the ultimate backdoor, in my opinion. I've managed to push all the way to an inhibitor in the first 20 minues of the game, and can usually take out a good number of turrets early-game.
I found that Kayle at L11 and Trist at L11 are capable of backdooring turrets.
Kayle ults on herself while Trist buffs with Q. Both attack and turret dead if Trist has carried properly up until that point and Kayle has worked on a decent Tank/DPS build.
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