ok heres my point of view:
Squall>cloud
Squall>seifer
cloud>sephiroth
now we all know seifer is one of the worst villians there is right? well maybe it just appears that way since if Squall used lionheart he would be superiour to cloud, and Cloud is superiour to Sephiroth. So maybe Seifer could beat since he had to put up a fight with someone who is superiour to cloud who is superiour to Sephiroth. Though then remembering theres more than one sephiroth so that can multiply his power, the size of his blade is incredibly lond so he can use long distance techniques and the fact that when he revives he becomes stronger. Plus seifers to stupid to be able to beat Sephiroth. So Sephiroth should win.