Yunalesca and Evrae were pretty hard. Yunalesca killed me once, and Evrae killed me about three times. You have to have either really strong characters or an awesome strategy.

Sin, Braska's final Aeon, Seymour Omnis were all very easy. I thought fighting my own aeons would be tough, but it was over in one turn for all of them. Sheesh. Oh and Yu Yevon, ah I love my Auron and his nifty little zombie attack.