Alright, diving into this because I've spent way too much time looking for exactly this kind of stuff. The request for 'badass powerful Harry' can go a few different ways, and a lot depends on whether you want him to earn it through grueling work or just wake up with it. I tend to prefer the former, where the power feels grounded in the world's rules.
For a truly methodical build-up, 'Harry Potter and the Prince of Slytherin' is a beast. It's a massive series that completely reworks his childhood and introduces a ruthless, intelligent version of Harry who is sorted into Slytherin. His power isn't just raw magic; it's political maneuvering, ancient rituals, and a frightening grasp of magical theory. The badass moments come from outthinking opponents with terrifying precision. It does get incredibly dense with world-building, though.
On the opposite end of the spectrum, there's 'Harry Potter and the Methods of Rationality'. Love it or hate it, this Harry approaches magic like a science project. His power is intellectual arrogance and a relentless drive to experiment. Watching him deconstruct magical laws to create overpowered solutions is a specific kind of satisfaction. It's less about epic duels and more about exploiting logical loopholes until the setting bends. Definitely not for everyone, but it defined a whole subgenre.
If you want something more traditional but with immense payoff, 'The Denarian Renegade' series by Shezza is a wild crossover with 'The Dresden Files'. Harry gets bound to a Fallen angel, gaining access to hellish power at a terrible cost. The fights are cinematic, destructive, and morally complex. He's powerful, but it's a corrupted, desperate power that constantly threatens to consume him. The series is abandoned, but what exists is a thrilling ride into dark urban fantasy.