For Kingdom Hearts it was the Stealth Sneak in Deep Jungle (man that drove me crazy!), Ursula the first time cause I didn't know to attack her cauldron, and Malifecent Dragon. Man! That took me the most tries out of any other boss battle put together! But now she's super easy. Funny ain't it?
For KH:COM it was Hades , Riku for the fourh time, and Marluxia the final time.
For KH2 its the toughest battle is keepin' myself alive waiting for it! LOL!