Graneries only help to make one's city unhappier, faster. I find that most of my cities will stagnate due to unhappiness at around 7 population, in the early game.
With my capital, it really just depends. Sometime's Stonehenge if I don't have creative, but usually it would be a barracks.
Early on, the next 3-4 settlements will have an Obelisk placed in there to expand their borders unless I have Stonehenge or creative. If I do not need an Obelisk, I'll purchase a barracks