Riven and Zyra
I always hate picking a lane as Riven and getting Zyra. Well I hate Zyra in general cause no matter who you choose, 9/10 times you end up feeding her. She tore my Riven apart not only from the start, but pretty much every time I continue to encounter her. While I last a good 10 or so seconds against other champs, even with 90 some odd magic resist, and 1900 bonus health from items, she tore me down in 2 seconds without even touching an ult. Riven's shield is completely useless short of using it as a flash now.
What adds to the frustration? When players at the end insist you suck because they refused to do a simple lane switch and you come out with 10 deaths and only 1 kill because no one else dared to visit the lane to help.
I'm not claiming to be master fu, I just got Riven a couple of weeks ago and still stick to AI games. But really, don't open your mouths and run off with insults when I'm absolutely certain Zyra needs a dmg nerf :P