What color you want is purely personal preference. No betta you buy at a LFS will be show quality anyhow, so that really doesn't matter. Just find the betta that you personally think is pretty, and that will be your perfect betta.
Some color are more common than others, simply because the are more dominant colors. Blues and reds are your most common, although cellophanes are also becoming pretty common in some stores, so are cambodians. Bettas with marbling patterns also aren't really hard to find, neither are bettas with butterfly markings, although I'm not sure you can call many of them true butterflies considering how jagged and incomlete the lines often are. They make for some interesting effects regardless of 'strain purity.'
Black, true green ( a true green is a dark forest green color without any blue) and opaque whites are generally considered some of the rarer colors in bettas, but rarer does not necessarily mean better. Black melano bettas are often sterile, true green in a betta is an extremely reccessive gene, and opaque whites often have other color washes on them that make them look more pastel.