The Following content is an opinion
Do not allow it to offend or cause problems, such as flame wars. These opinions are also not the official opinions of Final Fantasy Answers, which can be taken to be a neutral point of view.
In Dissidia, the only game where they all come together, they try to balance everyone out, though some actually have a little more potential power than others, it really comes down to how well you use the character, not which one it is. If you don't like that answer, well its really hard to say, there are a lot of things that are completely relative in the Final Fantasy worlds, say the Ultima Weapon, it has three incarnations, all completely different, so they can't even be used as reference points. The sources of magic are different(i.e. Materia, Magicite), a lot of people reading this are going to be angry for what I say next, the only real way to decide that, and its still not 100% accurate, is by going off of the statistical numbers, and through that, Tidus beats everyone hands down, simply due to the ability to break the number barrier that every other main character from before has been stopped at, i.e. reaching 99999 hp instead of 9999(the cap from before), damage to match that, and so on. Though some would say that he shouldn't really be counted as the strongest, as if the barrier was removed from other main characters, they could all reach the same level of power, but then it'd be unanswerable again unless you have some sort of bias towards a character.
The case for Tidus can also be argued for Lightning (Final Fantasy XIII) due to the normal damage limit being 99,999 (the max in FFX) and with a Gengi Glove equipped, she can reach 999,999 damage.