Give me a list of the qualities that each girl you like has. We'll call them "girl 1"( the one you dated) and "girl 2" (her best friend). Also, when you say you "dated one", what exactly does that mean to you (went to the movies a few times? were you "exclusive" meaning you called her your gf and she called you her bf? how old are you? and how long ago was it that you dated? why did the two of you stop dating?) If you answer all of these questions I will be able to tell you exactly what you should do. : )
I think you should stick to just one, if you are already dating one stick to her. It is best that you be honest to both so that they can understand you sometimes. Try to avoid the other girl, do not think about her and give more attention to your girlfriend.