Look, forget the huge epics for a second. The promise that gets me isn't some grand magical vow. It's the quiet, desperate ones in private fics. There's this one, can't even recall the title, where Draco swears to Hermione in a shuttered room at Grimmauld Place that he'll make sure her parents remember her, no matter what the war costs. It's not a spell. It's just his word, and the fic spends chapters showing how that single promise reshapes his entire path—he starts studying obscure memory charms, he lies to his own family, he risks everything on a thread of hope he gave her. That kind of promise is heavier than any Unbreakable Vow to me.
Those big ritual scenes in fics like 'The Auction' or 'Manacled' are incredible for drama, sure. The magic is visceral. But a whispered 'I will come back for you' in the dark, with no witnesses but the characters themselves? That's the stuff I save in my bookmarks. The weight comes from the trust, not the magic. It makes the eventual payoff, or the heartbreaking failure, hit so much harder because it was a choice, not a compulsion.