I think that there can be comparison's based on their most impressive acts, at least for the most part.

Due to the Shenanigans of FF VIII and the bizarre nature of Time Compresion and the seperate things needed to do it, I have no clue how to gaige the power of Ultimecia in relations to others.

Kefka, on the other hand, could on a whim wipe out entire cities, and at the end it appears he is able to do that to pretty much everything as he threatens to do so.

Kuja is comparable since his Ultima attack actually is able to also casually wipe out the face of a planet.

If Sephiroth had that kind of Power it's not likely he would need to summon meteor. Meteor, however, takes the seven days to summon, where as the other two can seemingly do their powers at will. This would leave me to placing Kuja and Kefka over Sephiroth.

I want to say Ex-Death may be the strongest, as he's sucking people into a void of nothingness, and is the actually able to kill off party members permanently in the last fight, though this is another one I don't know if that's entirely efficient way to describe it. I would certainly put Ex-Death over Zeromous in terms of power though.