Season 2 is my favourite season with Season 5 in second place.
I loved the Angelus arc. Buffy was the first proper show I ever watched (I was 7 when it premiered) and seeing Angel go evil and hurt Buffy and everyone was awful and painful (in a good way of course).
My favourite episodes are Passion, I Only Have Eyes For You, Becoming, Lie To Me, Killed by Death, Surprise and Innocence,