Liquid egg whites are one of the best sources of protein available. They are cheap, available almost everywhere, and are nearly 100% pure protein. There is really no such thing as the "best" type of egg whites, because they are all similar and all have nearly the same nutritional content.
As the name implies, liquid egg whites are simply the "whites" of the egg. The yolk is separated out on a conveyor and the whites are then pasteurized, packaged, and sold. Some brands of egg whites are flavored, which makes the typically low-on-flavor product more appetizing.
Egg whites are popular in the bodybuilding community because of their high protein content and minimal calories, fat, and cholesterol. Egg yolks contain large amounts of fat and cholesterol, which makes it much healthier to eat just the whites. This also makes them popular among those who are health conscious and those trying to lose weight. A high-protein diet is beneficial to many people, and egg whites make it easier to meet protein needs.
Liquid egg whites are much more convenient than buying and separating conventional eggs. They are typically sold in resealable bottles and can be poured straight into whatever they need to be in, as opposed to the tedious process of cracking eggs and separating the white from the yolk. Many bodybuilders eat upwards of two dozen eggs a day, so using this product instead of whole eggs saves time. Cracking eggs can also be a messy process and pieces of the shell can end up in the whites.
Unlike most normal eggs, the liquid whites are pasteurized, which means that they are also safe to drink raw. While normal eggs may be contaminated with salmonella, the pasteurization process kills the bacteria in the liquid. Many athletes and bodybuilders drink raw eggs because they are convenient; cooking the eggs may also break down some of the nutrients that they contain. The eggs or egg whites can either be blended into a shake or be consumed alone.
Although there are many supplement companies that label their egg whites as protein supplements specifically for athletes, these are not necessarily better than those available in most grocery stores. All liquid egg whites available in the United States are pasteurized and approved by the U.S. Department of Agriculture (USDA). They all have essentially the same nutritional content, so your choice should be based on availability and cost.