Ormund Baratheon

Lord of Storm's End and, in the long view, the most consequential Baratheon before Robert himself — not for anything he did in the field, but for whom he married. His wedding to Rhaelle Targaryen, daughter of King Aegon V, crossed the two lines for the first time in three centuries.

That single marriage is the entire legal basis of Robert's claim to the Iron Throne. Without Ormund's wife, the rebellion is a rising by a lord with a grievance; with her, it is a cousin of the royal house asserting a right. The distinction mattered enormously to everyone except Robert, who preferred the version involving a hammer.