Avoid dark beers like stouts. Their flavor and color are too strong for the batter. Light beer, like pale ale, is best. The purpose of adding the beer is really the yeast and carbonation, which help make the batter crispy and puff up - not the way it tastes. The alcohol itself cooks off in the hot oil, much like cooking with wine. For that reason, there's no need to choose an expensive can of beer. Just grab one that you'd usually drink, as long as it's a light beer. Whatever brand you do choose, just make sure it's very well chilled. This is important to create the light, crispy breading.