Square/Enix/Square-Enix, thanks to all the Final Fantasy titles, not to mention Legend of Dragoon, Chrono Trigger, Secret of Mana, Saga Frontier, Dragon Quest, and Breath of Fire titles.
I'd like to say Bethesda takes second but their library just isn't big enough to compete with Nintendo, I've been buying from them since the NES, so.. many.. Mario Games.