What kind of prom dress looks best on a 5'9 sophomore with long dark brown hair and eyes golden skin and have small boobs?

There are a lot of options, it all depends on your taste. However, I personally love the look of a knotted-top dress for taller girls with smaller busts.

However, there is such a wide range of dress options that you may have to think a bit more about what you'd personally like. Do you like your waist/have more of a nipped in waist? Then go for something that emphasizes or does not float over the waist. If you're indifferent about your waist, want to disguise a belly, or have more of a straight midsection, empire waists look great. Your bust is small, so you could definitely get away with plunge options, just making sure to keep to the rule of only showing two of the three at a time: bust, legs, back. Are you more into showing your back than going into an embellished front? The other way around? Do you want to show off your legs? And etc. etc.

As for colors, I always think that golden skin and darker features look great with rich colors (coral, emerald, olive, rich greens, navy, burgundy/plum, red, etc.) Again, it's up to your personal preference.