I generally think these kind of polls are silly nonsense, because specific game mechanics and each game's unique setting make a 1:1 comparison nearly impossible. Hell, someone here already mentioned Sabin suplexing a train because the game mechanics let him suplex stuff and you fight a train, while in reality he may not be anywhere near that actual level of strength. Similarly, Bartz being able to change classes being used as an argument because he has the ability to do literally everything in terms of magic, healing, support and attack types, and even Cecil being a knight in a high fantasy setting that's somewhat realistic being compared to Tidus as a peak athlete that got even stronger in a setting that is somewhat beyond strict realism. Hell, even trying to compare magic ability when Cloud has access to materia that let's him use as much magic as he has MP for, where Squall has to draw magic and can literally run out with no way to replenish mid-battle makes using these kind of mechanics kind of disingenuous.
Mechanics aside, I'd say the best way to determine who would be "strongest" of the bunch is by looking at story-based, out of battle feats. Things like Squall and company being able to take down a being who absorbed all of time and space, or Cloud literally slicing an entire skyscraper in half come to mind. With all that considered, I waffled between Cloud and Squall, but had to come down on the side of Cloud, because of the aforementioned ability to slice literal buildings in half. If you're gonna argue your case using in-battle mechanics then it's a never-ending and unwinnable fight, because all of them can summon some manner of planet-obliterating orbital laser dragon in some manner or other in the later entries, and I'd wager that some of the earlier ones intended various dragon summons to be of similar power and scale. If we have to base it off of canonical feats of strength, I've yet to see anyone aside from Cloud pull something that ridiculous off. And I'm not even the slavering Cloud fanboy that a lot of FF7 fans are.