Season 3 for me, everyone grows up and becomes more mature. Relationships change, are tested and get stronger as a result. Every character (and there are a lot of them in this Season) gets something to do, which is a testiment to the writing quality.
There are so many good episodes, and maybe like 2 bad ones (the first two in my opinion), we find out about the Mayor and how Sunnydale came to be, the mythology of the show starts to get built here (The Watcher's Council, Angel getting his own show, Faith the dark slayer, the intro of Wesley), this has the beginning of so many great things. 🙂
I really like other series (5,7 and 2 particularly) but this has a consistantly good balance between stand-out lone episodes and a great season long arc- all the other seasons tend to have one or the other.