Alright, let's break this down. If you're hunting for 'My Hero Academia' harem fics centered on Izuku, the scene's pretty fragmented. A ton of the really popular, long-running ones live on 'Archive of Our Own' (AO3). The tagging system there is a godsend—you can filter for 'Midoriya Izuku/Everyone' or pile on specific character tags. I've noticed the writing quality on AO3 skews higher, and the comment culture is less... chaotic than some other spots. But don't sleep on FanFiction.net (FFN). It's older, the interface feels it, but there's a huge back catalog of classic harem fics from when the fandom first blew up. The algorithm for 'most follows/favorites' heavily favors those epic-length stories, so the top of the pile is usually dominated by fics with 50+ chapters.
That said, for a very specific type of harem story—the ones that are heavy on power fantasies, OP Izuku, and often crossovers—SpaceBattles and Sufficient Velocity forums are weirdly dominant. The discussion threads for those stories are part of the experience, with readers debating power mechanics and plot points in real time. It's a different vibe: less about the romantic slow-burn and more about the 'and then he got another quirk and another girlfriend' escalation. Personally, I find the forum format clunky for actually reading, but I get why writers like the immediate feedback.
I'd actually steer clear of Wattpad for this niche. The discovery is terrible, and the popular stuff tends to be... let's say, aimed at a younger demographic with simpler prose and more clichés. You might stumble on a gem, but it's not the first place I'd send someone. My process is usually: check AO3 with careful tags, then search FFN for authors I like from AO3 to see their older work, and finally lurk on SpaceBattles if I'm in the mood for something unapologetically over-the-top.