What is the most prominent moral put forth to the audience in HP?

For me, it's a cross between 'Love defeats evil' and 'Always stand up for what you believe in'.

Love is the strongest weapon and conquers all.

Love is the strongest magic, and that there are worse things than death and pain.

In my opinion, it's "look beyond mere surface" or "don't be clouded by your prejudices".

A lot of the main plot turns revolve around the biased opinions people have of each other (well, mainly that Harry has of others), such as Harry's shock at finding out that his dad might not have actually been such a great person after all, and his hate towards Snape which turned out to be unreasonable. In contrast, he idolizes Dumbledore and Sirius even up to the point of ignoring their flaws.

Also, the entire degrading attitude towards Muggle-borns and the war between races; all of this is a moral take-on on discrimination.