Well I went with Cloud.
Celes is one of the best characters no doubt, her rescue, the three conversations she has with Cyan, Edgar and Terra, Opera, dealing with Setzer, her actions on the Floating Continent, start of the World of Ruin, she has many many great scenes. The Crescent Island part was pretty weak for her, being mute and being shown in fallen to her knees animation quite a bit, but other then that, she was an awesome main character.
But I like Cloud better. We see him at every stage of his life starting from age of 7, his character arc takes you every which way, the fact that it was originally intended for him to really not be the real Cloud and be Sephiroth's shadow helped perhaps, cause they pulled no punches in breaking down the trust in his character before building it back up.
After everything that happened, Sephiroth being set up as this unstoppable force, taking Cloud from SOLDIER first Class to a nobody that serves as Sephiroth's puppet, the scene where it turns out he really did save the day, save Tifa and Zack and defeat Sephiroth all those years ago is perhaps the best scene ever for me.
Advent Children damaged his character, but coupled with On the Way To A Smile, it helped to make him more of real character with real problems. He was happy, he moved on with his life, was supportive to Tifa, co-partnered starting a business, then started another, that let's him utilise his fondness of motorcycles, then he was asked to visit a fallen loved ones grave, he got the responsiblity of this inevitably dying child thrist upon him, and try as he might to find a cure, there isn't one and then he started dying too. All this was too much for him until Tifa, Aerith, Marlene and Vincent helped him.
The only real mark I have against him is how Advent Children Complete changed his scene before defeating Sephiroth.
Sephiroth: Tell me what you cherish most.
Cloud: There isn't a thing I don't cherish Zack and as Zack's living legacy, there isn't a thing I don't cherish.
I know SE loves Zack, and they admitted to seeing him as the perfect man, but that took it to far for me.