Well like everyone else(mostly) i'm voting for cynthia. Her team in DP were rediculously powerfu(pokemon, move and level wise) and it took me quite a few times to beat her. She was slightly power down in platinum but it was still very difficult...and frustrating *shiver*. But when I beat her it felt like a great acheivement. After her i'd say Blue for my FL/LG run. I dont think I ever fought steve :/, wallace was easy for me cuz of meh Manetric, lance was somewhat of a challenge tbh, and the only difficult part about about Iris was Haxorus.
....Oh yea Aldar. Yea he was no challenge at all and looking at the poll I view it hilarious as he has not one vote.
But again, Cynthia was hands down the true champion...until I beat her bout 4 restarts later.