I'd have to say the beginning of season 2 as well.
Though, I must admit, that's tough for me. I kind of put a lot of weight into Buffy's speech in Prophecy Girl about being 16 and not wanting to die, since that was the moment that I really, truly decided that this show was capable of some really great things. I liked it before. But I think that's what hooked me, the first time around, with my first full watch of the show.
But season two has so much going for it. As previously mentioned, Spike and Oz. Giles and Jenny. When She Was Bad. Yup. Good stuff.