Seeing people ask about platforms for this pairing always makes me jump in, because it really depends on what kind of experience you're hunting for. My main haunt has been Archive of Our Own, no question. The tagging system there is a lifesaver, letting you filter for specific dynamics like 'Bakugou Katsuki/Midoriya Izuku' and then sort by kudos or date. It feels like the archive was built for navigating massive fandoms like 'My Hero Academia'.
That said, I've found some absolute killer fics on Tumblr that never got cross-posted. Writers there sometimes do shorter, more experimental pieces, or thread fics that evolve from asks. The downside is the discoverability is a nightmare; you have to know the right blogs or get lucky with a reblog chain. And let's be honest, a lot of it disappears if someone deactivates.
I used to read on FanFiction.net a ton, but the content for this ship there feels... older? A lot of fics from the early seasons, and the interface makes it harder to sift through. For sheer volume and the ability to really drill down into what you want, AO3 is still my top recommendation, but I'd never tell someone to ignore the wild, creative, and sometimes fleeting gems on Tumblr.