You're beatings only make me want to steal it more! :D
For the topic, I assume we are talking about old school FF ne?
Let's see...
I think I'll have to go with Zeromus as well. He was the only boss that was actually difficult. It took some real good luck to beat him. He was just so god damn fast! Or perhaps my characters just sucked..:-0
Mind you, I haven't beaten FF3 yet, so I suppose that boss could turn out to be more difficult than Zeromus.