So many choices...
I'm going to part ways with the majority and say The Zeppo. Which is weird, because I am not really a Xander-phile. It's definitely not critical at all to the larger arc of the season, but I really view it overall as the most complete, polished and well-made episode of all of S3. I personally can't fault it for much of anything. It was either this or Amends, I think, for me (Amends touches my vulnerable soft spot for a good love-centered story...Bangel at its best). And S3 is not all about Faith and the Mayor for me either, although their great.