it has been recently prooven (and was on the news) that drinking normal milkshakes is just as beneficial as weight gain shakes, so i recommend those.
and try to keep your daily calories as high as possible, 3500 to start with then eventually 4000 calories, if you can. you need to gradually eat more to increese your stomach capacity.
best of luck.
For fast weight gain, try protein shakes and specialty foods geared towards bodybuilders in the 'bulking' phase, as well as refined carb-heavy foods like pasta and bread, and high fat snacks like cheese and preserved meats.
I also highly reccomend peanut butter. Add three teaspoons of peanut butter a day to your diet and you'll be heavier in a flash.