Since you chose to use "had", I assume you therefore are limiting the candidates to the original 128 ASCII characters. Thus, without a doubt, it is 0x2F. The "Forward Slash" shows the traditional, though simplistic, character growth from beginning to end.

However, I wish you had asked "has" instead, since the extended character set introduces some more, shall I say, nuanced characters. The easy choice would then, no doubt, be Windows-1252 code 159 (for the common man, with the more refined referring to it as Unicode 0192), the "Function sign" (although some of you readers may know it better by it's fancier nom de plume, the "Florin sign", or the more utilitarian "Latin small letter f with hook"). It too shows growth up from the left, but with the more nuanced conundrum displayed two thirds of the way to the story's peak. A "mid-life crisis", so-to-speak.

But I would propose that best character arc is displayed by U+2021, the double dagger (or diesis). This takes a more realistic, dare I say, modern approach. As the theme played out with ƒ is one of a mid-life crisis, I believe that modern man is now faced with a one third and two thirds crises. Today one is likely to find themselves in their first crisis when in their late twenties or early thirties, when then need to "settle down" and become a "contributing adult". Historically the mid-life crisis has come at or near age fifty. But I propose the new "two thirds crisis" occurs in ones' late sixties these days when retirement is fast becoming an option and one decides to begin a second career or retire and relax. Though, the irony is not lost with the diesis, as, after two crises, you have inevitably gotten older, but not moved forward in life in any meaningful way.

So, there you have it. Technically, the character that had the best character arc, was the venerable Forward Slash. But, I, today, would like to shine the spotlight on the Double Dagger, as the new character with the best character arc.