Archers, at least three of them. And, obviously, a warrior. Even if the city has walls, in the early game, with three archers and a melee unit you can capture it without a problem.
A little later, but still in the ancient era, I will use a battering ram, an incredibly useful and often neglected support unit, two or three melee units and the three archers from before. Warriors with Battlecry and a battering ram are pretty deadly to city walls, even though they receive some damage attacking.
Then, in the classical era, I would use two or three catapults, which make devastating damage to the city walls.