Top - Darius - Good luck. Endless harass with super sustain, the nastiest AD steroid in the game, and 1200 true damage chainable nukes while building full tank. Strong all stages of the game.
Jungle - Rengar - I don't play squishies all that much, but seeing him delete your carry in half a second is so annoying. It gets to the point where all your peelers have to hug your carries or risk them getting instagibbed. Snowballs incredibly hard too.
Mid - Talon - Yes. His snowballing is absurd and you are lucky if you don't die to his lvl 6 ignite all in. If you beat him in that duel than sure you won lane, but that doesn't mean he can't cheese again. He also gets super armor pen so you won't be surviving. I was 10/0 on Vel' Koz against Talon (don't ask) and yet he proceeded to 100-0 me despite his terrible CS/KDA.
ADC - Vayne - Lategame you die in 3 shots, her %health true damage is absolutely insane, and she can basically have 3 BoRKs. Also extremely safe laner thanks to her kit, and her early game isn't exactly weak.
Support - Soraka - her healing is absurd and very annoying to fight against due to her poke and sustain. Her silence is also quite annoying.