Nestle Alpen Blend European Style Drinking Chocolate is one of my favourites. I find a lot of drinking chocolate to be too sweet and not chocolatey enough. Many have a gritty texture too.
This product is smooth, it has a darker taste than some other brands which is great for me because I prefer darker chocolates to milk or white. I think it is still technically a milk chocolate mix though.
Another good thing is if you prefer milkier chocolate you can just add fewer spoonfuls of the mix.