I know I'm going to sound like a butthole for saying this but IMO the two best supplements are a blender and a power rack. To gain weight, some say take your weight and multiply this by about 25 so that would put you at 3250 calories. Are you eating this amount, can you eat this amount? Can you eat more? You'll have to in order to keep gaining weight. That's where a blender would come in handy, liquid calories are great when you can't stuff more food down your throat.
A power rack is great to enable you to train hard on squats, presses, then add in deadlifts, chins, dips, etc. Search out something like Starting Strength or Brawn or Super Squats.
Creatine can help you gain weight, so can weight gain supplements and protein powders but not in and above themselves. Anabolic halo or other junk like this is to be avoided.
Take in enough calories, train hard on basic compound movements, get enough rest, look both ways before crossing the street, don't run with scissors, etc.
Hope I've helped