What product is best to gain weight?

I weigh 100 pounds and I'm 19 about 5'6. Iv been trying to gain weight and get up to 110 or 115 again because I dropped to 90 pounds when I was 17 (not anorexia or bulimia just stress and bad eating habits) but it's taking alot if time for me to gain it back because of my metabolism. I eat alot more now and I take calcium and fiber pills every night just to help my health. I just need something to help me along with gaining weight back.