"Healthiest" alcoholic drinks?

What cocktails/drinks are the least fattening (i guess you could say lol)? My friends invited me to a happy hour tonight and I just turned 21 recently, so I don't really know much about the nutritional content (calories, etc.) of most alcohol. I like rum and coke, maybe ask for it with diet coke and I'll be ok? Or is something like sangria better? Light beer? Thanks!!!