I heard once that Cecil is the strongest swordsman in the series but is he the strongest hero? I think it is Cloud although I haven't finished FFIV and there are many people who can actually rival Cloud. Any who do you believe is the strongest hero?

Rather a ambiguous question isn't it? All the characters have the potential to be strong/powerful. Example, in Final Fantasy VIII, junction 99 Ultima's to the STR stat, increases the characters strength rather substantially (I'm not sure as to what exact amount though). Also take into account weapon and armor equipped as well as magic/Materia and it's really up to the player how they go about setting their characters up.

Of course each player has their own personal favorite over who's the strongest though. Personally, I like Edgar from Final Fantasy VI due to his Tools ability.

I don't think its really a fair question. Most of the final fantasies have different growth, for me its Terra, my Terra does max damage with both physical and magical at level 70.