I've never been a big fan of the "strongest orc" as just a slab of muscle with a magical weapon, which is what a lot of cheap harem stuff does. The truly formidable ones in the stories I click with are defined by a contradiction. They have this overwhelming physical or magical power, sure, but the compelling part is how that power is tempered, or even complicated, by something else. It might be a rigid, almost archaic code of honor that puts him at odds with his own desires, or a deep-seated protectiveness that borders on obsessive. His strength isn't just for conquest; it's the foundation for a specific kind of stability he provides, making his territory or his clan a sanctuary. That creates a more interesting dynamic than just winning fights.
What really separates the mightiest from the merely strong is how his power shapes the harem dynamic itself. It's never a democracy. His will is the central gravity, but the interesting authors show how each partner relates to that gravity differently. One might challenge it directly, another might seek to subtly influence it, and a third might find profound security in simply orbiting it. His strength defines the rules of the game, so to speak. I remember a web serial where the orc chieftain's primal aura literally calmed the magical storms in his territory, which became the reason displaced fae and humans sought his protection—his power created the conditions for the harem to form organically, not through random seduction.
Ultimately, the trait that makes him 'mighty' in a narrative sense is narrative dominance. His perspective, his goals, and his internal conflicts drive the plot forward more than the collective wishes of the harem. The story is about his journey, and the relationships are facets of that. If the story becomes just a checklist of romantic acquisitions, the orc's might feels cheap. But when his strength—whether it's battle prowess, political cunning, or a terrifying ancestral magic—continuously alters the landscape of the story, that's when he feels truly formidable. That kind of character makes the 'happy' in the harem feel earned, a hard-won peace he carved out himself.