I voted other. I think there were 3 characters that deserved to kill Sephiroth.

1: Cloud obviously--The mind control, the pain, the suffering, the putting him in a wheelchair all crazy (well, that was the lifestream really huh), the having him almost kill his g/f, etc

2: Tifa--1st he shanks her after she graciously guided him through the mountain. Then he kills off her family and townsfolk. Then he messes with the dude she loves, and to top it all of, shes really passionate about savin the planet.

3:Aeris--Dude, he killed her! Theres more but ill save it

and why i picked other:
Professor Gast deserves to kill him, well 1st Hojo then Seph
Vincent deserves to kill him
Any Seph clones deserve to kill him
Clouds mom...just because I'd like to see her lay the smack down
Bugenhagen deserves to stomp his ass

And most importantly...THE PLANET should be the one who kills Sephiroth. How that would work I dont know, but the planet should smote him...or any of the weapons that protect the planet