Best way to cook rice to serve cold later?
You want to use oil as just any fat.  More to the point, you want something that is liquid at refrigerator temperature.  Butter becomes solid at that temperature and makes for a terrible texture.  Beyond that, err on the side of using an overly flavorful broth, as flavor is far more subdued at colder temperatures.  Beyond that, to ensure tenderness cook it slightly past al dente, as the texture will firm up as you cool it.