Rengar was able to one shot people with 2 items: Trinity force and Deathfire Torch
About 1.5-3 years ago, then ap Rengar was a "real" thing. With full ap build he was able to destroy towers in a few hits, one shot squishies with only two items, Trinity Force and Deathfire Grasp, without any countery play, besides Pink ward. Riot nerfed him after years only.
http://www.youtube.com/watch?v=qVKlqIe0HvI&t=1m35s
We had ap Master Yi 3 years ago. He was able to 1vs5 whole enemie team and could heal for absurd amounts of Health. In video he looks hilariously and I have no clue how was he allowed to do such stuff for pretty long time, comparing him with other champions at that time.