I haven't played the PS stuff, so I can't really say there.
But I do recall FFIII Dark Cloud doing nothing but an ultimate attack over and over again. That was hard as a mug. But I also recall giving up on beating Ex-Death with any fairness and simply getting rich and Coin Tossing him to death. (I later leveled a lot and found enough skills to beat him proper. But now I don't remember what they were so I'll have to play it again someday. Er, maybe... Maybe it'd be a waste of time.)

List:

5 Ex-Death
3 Dark Cloud
4 Zeromus (Right?)
1 Chaos
2 Emperor
6 Kefka

Kefka was ridiculously easy and the FF2 boss hardly hit me before he died. FF1 is difficult occasionally, at least. But not very often. Still easier than 4, maybe. I don't recall having a hard time with Zeromus, but I recall the monsters in FFIV being really tough, so I imagine that it was harder than the other 3.

Ironically, I think that Kefka is the most badass boss character-wise. Just not very difficult in the game. Probably because there are about 12 or 16 or so party members that you can use to kill him with before you go down.