We weren't just the 1st Legion. We were THE Legion. The only one of its kind. 10,000 of the Emperor's best and brightest. The first to stride into the stars to reunite humanity. Long live the Lion.
Hate to be -this- guy, but the emperor himself stated that Horus was the first among equals. Thats why he was chosen to be Warmaster. He was acknowledge at being the best of the best, even by his brothers (at least most of them, *cough* Angron *Cough*) so that should be that.
He wasnt chosen because he was the "best" but because he was the only one with the rest would follow (at least the majority ). Ferrus Mannus,Angron,Perturabo etc.etc!
Well, that point is debatable, as I've heard an argument from most of the other primarchs as to WHY Horus was the best, but all that aside. It doesn't change the fact that the emperor himself chose him, and few can argue with his choice since they are HIS sons
Horus actually became somewhat of a god himself, he would have been able to control chaos and become the new emperor.
Horus himself thought that Sanguinius was his equal in everything but was better at forging relationships with others.
