It depends on the situation. If I want light beer that'll leave room for a big meal, then I go for Blue Moon with a slice of orange. It's light yet still smooth and not bitter. I hate bitter beer.
If I've already eaten and am not full or if I'm just snacking on bar food while watching a game or something, I'll go for the darkest, thickest beer the place has. If they have Guiness on tap, I'll do that (not in bottles nor cans as it just isn't the same). If they don't have that, I'll go for the darkest micro-brew they have. There is a micro-brew in Vegas where I live called Barley's that has an excellent dark stout. It's as dark as Guiness with a nice thick foamy head yet it doesn't have the same "burnt toast" kick to it that Guiness has. It tastes more like home-made dinner rolls and is nearly as filling.