I can't really vote here. Ghirahim and Demise both serve this role in different ways. I would personally describe Ghirahim as the
main villain, while I would describe Demise as the
ultimate villain. (I still didn't vote Ghirahim though, because that's my wording and it's misleading.) Basically Ghirahim is the primary, villainous presence throughout the game. He's the real antagonist, the enemy, the rival. Demise only shows up at the end, but he's actually built up throughout the game. Still, his presence as an actual antagonist doesn't really come until the end, but he's still the
Big Bad (source of all problems in the story), and he's still Ghirahim's master, end goal, and the final boss.
Ghirahim is the main villain because he's around the most and is the most focal of the two, but Demise is the ultimate villain because at the end of the game, he's the big guy, the one who was truly behind everything.
And besides, they're technically both two forms of the final boss (really, the final boss has three forms, the Horde Battle, Ghirahim, and then Demise). And even if you were to consider Demise the only final boss, it's still Demise wielding Ghirahim. You fight them both at the end.