Izuku Midoriya. Let me explain.
I'd start off in base key and start as Izuku whenever All Might says "become a hero."
Since Izuku began as an unathletic teenager, I assume that I can keep the stronger key, which is me. I'm able to use the workout routine more effectively and start off in beginning of series with a power boost. I also know how to use Full Cowling, so I start at a likely 8% power output. I'd also tell All Might about my prior knowledge and use it responsibly, only when thousands would die without it.
I'd conquer the entrance exams by using 8%, getting 2nd place in the rankings. I'd use AP shots to snipe others as well, and at the end against the Executor Bot, considering it's ranking (which I think 8-B would be more reasonable here) i'd use a Full Cowling 20% California Smash in order to defeat the bot in one punch.
From here, I effortlessly win/get top in most events until USJ. In USJ, if All Might loses I go full cowling 100% (similar to the Heroes Rising Movie) and absolutely destroy the USJ Nomu by breaking the ground, catching him off guard and then using Manchester Smash.
In the Sports Festival, I inspire Todoroki but lose the fight so that Todoroki see's that his effort was existent. In the Stain fight I carry due to my prior knowledge on Stain by using 20% AP Shots. Around this point I should have around 15% power output.
Final Exams work same way as in the real story. In the Training Camp Arc I mainly prepare for my fight with Muscular. I get to 20% and fight Muscular. I would use OFA at around 100% to one shot muscular, since he is, at maximum, Low 7-B while my OFA 100% should likely be comparable to Final Act Saga Deku.
Around this point in the Provisional Licensing Arc, I should gain access to the additional quirks. If Izuku was able to activate his quirk by thinking, "I need to use my quirk to capture Shinso," then it should apply here, where Danger Sense would be the first quirk to activate, as I would think, "I need to dodge these balls". After this I would realize that I have unlocked my quirks, and besides the second users quirk, i'd know how to activate them well enough.
I'd unlock Blackwhip in the Provisional Licensing Arc as well in order to effectively be able to rescue civilians. And when it comes to Deku vs Kacchan (if that still happens) Danger Sense one shot gg. In the fight against Mirio, i'd likely one shot Mirio, but due to his insane control of his quirk it'd take a bit for me to get the 20% hit off, but he can't do anything to me either due to my danger sense.
When fighting against Sir Nighteye, this is also a situation where Danger Sense makes me able to effortlessly beat this fight, as it takes away Nighteyes one advantage of being able to see into the future (as well as this, Deku's 20% key is Hypersonic while Sir Nighteye is Supersonic+)
In the Shie Haisaki arc, i'd likely gain access to 30% and then likely not need to go 100% in order to defeat Overhaul. Though, i'd likely do it anyways since it's an easy good fight for me if I do. The fight against Gentle Criminal results in me one shotting, but being careful to limit my power (if I went above 15% Gentle Criminal would likely die in one shot) i'd likely use Blackwhip to capture him easily though, and if he runs away, this'll be the unlock of float, which i'd use to gain flight.
In the Joint Training Arc, i'd absolutely carry my team, with myself gaining access to 35%. There'd be a situation to learn Smokescreen, and then i'd set off with 40% of One for All into the Meta-Liberation Army Arc. With my newfound powers and 40% of One for All, i'd tip off All Might about the villains plot. A team of pro heroes would be formed and the final battle of the series would begin.
Unlike the Paranormal Liberation War, Tomura Shigaraki would be unready, with the demolition of both the Paranormal Liberation and the League of Villains, but it's not that simple, even with High 7-C (should be comparable to pre-wiki update 45% Deku) levels of power. I would quickly be incapacitated by the fact that I would need to defeat Gigantomachia, even if it was only in one punch. I'd have pain manipulation (gained from the exercise to gain OFA, which is the reason why I chose to start from that point in time in the first place), so i'd be unable to use OFA as effectively.
From that point onwards i'd graduate, become the #1 hero and master 100% of OFA, as well as Fa-Jin and the Second Wielders quirk, avoiding the potential chaos, and continue living in that world. I'd not need to worry about finding a protégé, and when All for One breaks out, i'd one shot due to him being even weaker than his Weakened Form (likely). I'd literally be an unstoppable force. Heck, even if a character that's in a tier superior to 6-C came along and decided to commit genocide, i'd just store up energy with Fa-Jin before one shotting it.
And that, is what would happen if I was Izuku Midoriay.