I'd call Horus a leader.
He wasn't a scholar or an intellectual like Magnus or Lorgar, he didn't have the technical minds of Peretubo or Ferrus or the civic inclination of Guilliman. Of course he had a brilliant strategic mind, like all the Primarchs and was a great general but in their specialties it's likely each of the primarchs would have him beat. That doesn't matter though, beacsue they'd never go against Horus in the first place. He was more than a general or warlord, Horus was a leader, a monarch, a figure head.
He was charismatic, able to win others over and make those who served him love him. He knew how to delegate, to get the best out of people. He makes you want to follow him, you want to impress him, you want him to notice you. Sanguinius also has this nobility of spirit, this magnetism but Horus combines it with a sense of earthyness, he's 'one of the guys'. Effortlessly he's able to charm others and set them at ease, he becomes more than your commander or an aloof figure of awe, he's your comrade and friend.
It was Horus' brilliance in attracting and manipulating people which made him the best successor to the Emperor and was why half the galaxy rose with him.