How powerful should final bosses be?

I think that the final bosses should be powerful, but only as powerful as the story, or the game, allows them to be. For instance, I thought Jecht could have been much stronger. Sure it was hard without break damage limit and break HP limit, but when you get these abilities and level up so that you have 200 - 255 attack, you only need to hit jecht 3 times before he goes down.

Optional bosses should also be stronger than the final boss. Though it does seem wierd how something can be terrorising the world, while something stronger is lying dormant just waiting for your party to fight it. Like many have said, optional bosses are for those hardcore gamers how want more out of the game.

>>> I think FF bosses should be impossible to beat, enough to make you feel very frustrated..

I would want the final boss to be so difficult that I feel I accomplished something when I finally complete the game.

I hate leveling up but I want a hard final boss but not too hard. It would also be good if killng an optional boss would effect the ending or the fina bss battle. The optional hard boss might be main chracter of the story.