If I'd be forced to make a choice, then I'd say Pyramid Head, since I like the Silent Hill series more then the Final Fantasy series, but that's just me.

It's good to know Sephiroth won.

This is why Seph would win: even in his mortal life, Sephiroth was a legendary hero capable of doing many things many others could never dream. That Seph alone could take Pyramid Head, but coupled with the facts that when he comes back to become the Seph we all know and love, he become even better in every aspect except that he was an antagonist, and even in AC the ability to FLY, I'd say it's pretty obvious.

But IceDefenseGod, Pyramid Head could most likely take as many blows as Sepiroth could throw and even if he'd manage to kill Pyramid Head, he would come back a few minutes later, because.... (Spoiler!!!) if you see Pyramid Head in the first place, he's actually a part of you, so in a way... Sepiroth would be fighting himself... As weird as that sounds. (/Spoiler!!!)

Now I'm not saying Pyramid Head could win, what I'm saying is... It would truly be an endless battle since Sepiroth could most likely never manage to rid himself of the feelings that spawn Pyramid Head...

hey, if Sepiroth went to see a psychologist to get rid of these emotions, he'd technically kill Pyramid head.

well..not to contradict the Silent Hill reference DPumbliQ, but Sephiroth keeps coming back because of ol' Cloud's "darkness" (if u refer to the Kingdom Hearts Screenplay)....so saying pyramid head is gonna come back because he's a part of the you (or whatever protagonist of the battle) sephy 's gonna come back for the same reasons