Ok I know that Buffy hasnt been around for a long time, but considering the fact they are making a new film based on her, I thought there may be some fans of the show who still enjoying remeniscing about such a great show.
As for my personal favourite, I think I am going to have to say Once more with feeling stands out. The fact that a show such as this existed in the first place and actually became as popular as it was (and still is to many of us), is because of the fantastic cast and crew, the writers and actors who put the life into the characters pulled off many things that had never been done in a show that was based on a female character who fought vampires and other nasties, whilst dealing with the every day things a normal teenager would have to deal with. But Once more was the pinnacle of this in my humble opinion.
The whole cast sang or danced (even Allyson Hannigan who has said in interviews that she was a terrible singer), the whole episode was one hell of a journey, so many things happening and most of it was acted out through the power of songs. Several of these songs have found a permanent home on my music lists and I am not afraid to admit that I know most, if not all the words for every one of them. Highlights would include Spike's impromtu rock performance whilst trying to tell Buffy he no longer wants to be treated like an object, but he is still very much the "willing slave", and Tara's "Under your Spell" serenade to the Power of Willow, something that Joss Whedon was quoted as saying it was akin to musical pornography.
I just hope that when the new film is released, it does at least pay the highest homage to the television series, I am going to watch it and judge for myself, however I am definately not going to hold my breath in anticipation of it being a true "Buffy" movie. I hope I am proven horribly wrong.