Are you looking for meal supplementation or post-exercise recovery drinks?
Most "mass gainers" you will find are simply combinations of very high glycemic carbohydrates, protein, and maybe some fat. Quite frankly, they suck for so called "meal replacement." Eat them frequently and they will pack on the same kind of mass as load of sugar cookies and milk.
If you are looking for a good post-exercise recovery/anabolism stimulatory drink, many of these mass gainers fit the bill quite nicely.
If you are looking for what the current science says is the most optimal post-exercise recovery drink, shoot for something that has a 4 to 1 ratio of high glycemic carbohydrate to high BV protein. A good combo would be maltodextrin (which has the ability to rapidly restore muscle glycogen during post-exercise conditions without throwing out exessive free radicals) and a quality whey such as hydrolized whey, ion exchange, or CFM.
For a recreational athlete, 100 grams of maltodextrin and 25 grams of a good whey fit the bill nicely. For an advanced athlete engaged in high intensity training, double those numbers for 200 grams of maltodextrin and *50 grams of protein.
*Please note that 50 grams of protein as prescribed here WILL NOT be fully absorbed UNLESS a hydrolized whey is used.
Small amounts of fat may be used, but don't go overboard as they will slow the absorption of both the carbohydrate and protein - both of which your body will attempt to absorb like a sponge post-exercise.