Season 5. I haven't seen all of season 6 or 7, but season 5 was just so PERFECT (okay, the opener wasn't, but it brought the 'primal' theme.) The theme of family was so great, and "The Gift" brought together EVERYTHING from the season, which is partly what makes it the greatest episode ever (imo, of course). Other great episodes were Fool for Love, The Body, Intervention, and I Was Made to Love You. What I've seen of season 6 brings it to second, though, of course because of Once More With Feeling, but also Bargaining, Life Serial, Tabula Rasa, Smashed, Seeing Red, and Grave.