If you want to gain weight, don't use all those fancy products like the slim drink or the muscle milk. They will indeed make you gain weight, but be sure that it is not going to be lean mass that you will have.
For a healthy way of gaining weight that will be clean from fats and water retention, better use a good old whey protein. It is rather easy to choose a good one - just make sure it is made with whey isolate, has no sweetners, or additives. Personally, I would recommend the Isolation - it is 100% whey isolate (the purest form of whey protein), has no artificial sweetners, or any other stuff that you do not need. Yet, it tastes pretty awesome (especially the Colombian Espresso), and it has a very decent price tag.