*tries to predict the hardest boss for every disc (not including Ultima and Omega)*

Disc1: Brothers. There's hardly any really tough bosses on disc1 but I think the Brothers will be far from easy.

Disc2: Cerberus. For some reason, I've just got a feeling that Cerberus will cause some serious trouble.

Disc3: That Prototype-"something" that you fight right before the final battle with Seifer inside the Lunatic Pandora. Adel might be tougher, tough... but my bet's still on that Prototype-thingy.

Disc4: Ultimecia. It's pretty obivious.