So your happy with your weight? You say you have a hard time gaining weight... Have you ever really tried to gain weight? It really isnt' that hard. Just takes some discipline, desire and FOOD. Read the stickies in the diet section. They helped me gain 18 pounds in about 2 months.
rather than telling you to hit the diet and what not, which you should, I'll answer your question. I personally like pullups, bent over barbell rows, pulldowns. Pullovers and T-bar rows work well also. Check out exrx.net for any information on the exercises, along with pictures on correct form. Also what others said, just eat and eat and eat some more.