For me, when I think of "good acting" in a television series, I generally define the term as being able to show a good range of diversity in a performance, not just one type of acting, as good as it may be.
That's why I'm not picking S6. I thought SMG did an outstanding job, but her entire range that season, with few exceptions, was sad/depressed and angry. She had a bit of the old Buffy humor thrown in but mostly, that was it.
My pick is S3 (and that might be influenced by it being the one I've seen the most recently). Buffy goes through a lot that season, in a lot of different areas. It starts with her completely isolated and alone in L.A., progresses to her having to deal and move on from Angel, and then deal when he comes back and explore that angle. Her whole deal with having a rival almost equal to her in Faith had great depth to it, and that was something we really hadn't seen much before or after S3. The humor was fantastic and the way she acted with Giles and Wesley developed quite a bit, and moved the overall plot for those characters along really well.