"The Wish" is an interesting what if episode? Buffy is wished out of Sunnydale by Cordelia and chaos ensues.
"Helpless" is also great because it gives the audience a sense of Buffy would be like if her powers were taken from her. "Dopplegangland" takes a look at a vampire Willow and how much mess she can create.
The finale is also a lot of fun appealing to every teens dream about graduation. The story brings every character together in a fight of good against evil.
A very entertaining and sad series because it is the end of an era.