The snobby daddy's girl from Long Island was best known for her good looks. Throughout the shows lifetime, the culture basically changed their hair according to the new haircut of Rachel/Jennifer Aniston. The character of Rachel is actually kind of difficult to analyze. At first, she was the snobby, self-centered daddy's girl that had never worked a day in her life. However, as the show progressed, she had good jobs with Ralph Lauren and Gucci, she appeared to be relatively stable, and didn't sleep around like many others (although she had the only out of wedlock child of the show).
The Gospel according to Rachel is difficult, but I would argue it was...wanting everyone to like you. In one episode told Rachel that she was a pushover. She basically did whatever everyone else wanted to do and really did not have her own identity. This was especially true around her friend Monica who needed everyone to do what she wanted. The side effect was that nobody really knew what she valued, desired, or loved. She went with the flow and did not rock the boat. At the end of the day she will always be remembered as the prettiest one of the show, but know little more about her.
Rachel's gospel has increased our cultures value of not really believing in anything. If you believe in something and other do not like it, then you better not say anything. One better not disagree with someone or present a differing view or else they are a "bigot." Just look pretty, get the newest haircut, and don't rock the boat. This has big implications concerning ones faith convictions. If you have a differing view concerning faith or values, it does not mean that you have to be beligerant, but it does mean that you should bring up your thoughts and be bold in your beliefs (especially when it is concerning your Christian faith).
Perhaps, a close second would be her infactuation with looks, but this was not nearly as evident as her desire to be liked.