Finding stories where Slytherins rally around Harry really depends on what kind of dynamic you're after. Archive of Our Own is unbeatable for its tagging system—searching 'Slytherin Harry' or 'Protective Slytherins' will bring up thousands, but you have to filter carefully. I usually sort by kudos and then check the 'Additional Tags' section for things like 'Slytherin Solidarity' or 'Theodore Nott is a Good Friend'. That's how I stumbled on 'The Changeling' by Annerb, which isn't strictly Slytherin-protective but sets a fantastic precedent for Slytherin Harry narratives. For a more specific, curated list, the r/HPfanfiction subreddit has a few old but gold threads dedicated to the trope. People there genuinely debate the merits of fics like 'Harry Potter and the Welcome to the World of Grey' versus 'The Pureblood Pretense' series, which is a massive Slytherin-focused AU. My own favorite is a lesser-known one called 'A Necessary Gift: A Harry Potter Story' on FanFiction.net, where a Slytherin-sorted Harry slowly turns his house into allies through sheer stubbornness, and the protection feels earned, not automatic.
Honestly, the 'best' is subjective because the trope can swing from wildly OOC crack-fics to serious political dramas. If you want the protective aspect to feel visceral, look for authors who build the Slytherin common room as a character itself—the whispered warnings, the shared looks, the unspoken rules about not letting outsiders target one of their own. It's less about Slytherins suddenly being 'nice' and more about them respecting a different kind of strength or recognizing a shared enemy. I've dropped fics where Draco becomes Harry's instant best friend by chapter two; the slower the burn, the better the payoff. Sometimes the best protection isn't physical but social—navigating pureblood politics to shield him. It's that nuanced take, where Slytherin protection is a strategic, self-interested choice that later deepens, that really sticks with me long after reading.