KH1: Ice Titan: The fact I consistently had to stand around there and keep blocking the ice spears for like 20 years was a real nuisance.
KH2: Lingering Will: No explanation needed
KH358/2 Days: Never finished playing it, so I don't know
KH: ReCOM: Marluxia III on Proud: MY GOD, HOW I WENT THROUGH HELL!!
... I thought all three of those bosses were great.