I think my favourite part of the books is when Severus Snape shows of the dark mark to Fudge in the infirmary. This not only shows his character, that he is willing to do whatever is required to bring down the Dark Lord, but it also shows the forces for an against Harry. The Ministry represented by Fudge is willing to ignore the evidence before his eyes, Dumbledore provides quiet strength but ultimately, it is down to Harry who witnesses the event and realises that the fight is really on.
In the films however I think my favourite part is when the DA produce their patronuses. It's just beautiful.