Honestly, the fanon version of Severus Snape often tops the list for me, but in a way that feels more like an army of one-person guilt-ridden crusade than a group effort. He's not nice about it—the classic 'Snarky Bat of the Dungeons' who shields Harry through brutal occlumency lessons, subtle potions sabotage, and terrifying anyone who looks at the boy wrong, all while insulting his haircut. It's a specific flavor of protection born from a promise and self-loathing, not affection, which creates fantastic tension.
The Weasley twins are a close second for their brand of chaotic, preemptive defense. They don't just react to threats; they build them into their business model. Anyone who messes with Harry might find their socks permanently singing offensive songs or their hair turning a shameful puce. Their protection is proactive, public, and hilarious, which makes the Founders-figure or mentor types look almost stodgy in comparison.
Sirius and Remus are obvious picks, but fanfiction sometimes gives Arthur Weasley this quietly formidable edge. He understands the Muggle world as a weapon and the Ministry as a bureaucracy to navigate, offering a different kind of shelter. Still, for sheer narrative force, Snape’s bitter, relentless shadow-watching always leaves the strongest impression on a story’s plot.