There's always the sick joke battle in FFT against those 12 monks on grog hill.

I must say that Square doesn't do a very good job making their superbosses difficult. I've fought regular bosses in other RPGs harder than Square's superbosses. Most Square superbosses go down with 4-5 hours of post-game training. Maybe if they decided to let the US players fight the Dark Aeons I'd change my tune. The hardest Final Fantasy enemy is probably something out of FFXI that I haven't gotten to yet such as Kirin or Vrtra. Those bosses are so hard they take months of preparation.