quick, quick, before Extreme, Boditronics and Reflex post
Just kidding guys!
There's loads to be honest. Its hard to choose there are so many. I actually quite like Dymatize Mega Milk as a weight gain product because it has a nice protein blend and healthy fats. You can add carbs if you want them by getting some plain maltodextrin like Extreme Carbs, GN Complex Carbs, CNP Pro Fuel etc..
Reflex Instant Mass is a very good, more traditional weight gain product with a good protein/carbs mix. CNP Pro Mass is excellent also. People give maltodextrin a hard time as a carb source, but I think its fine so long as its not in massive amounts.