Six, with two as a close second.
I loved OMWF through Seeing Red. I could not take my eyes off the screen. I just kept popping DVDs into my player. OMWF was full of singing. I mean, come on! Tabula Rasa cracked me up. Smashed surprised the heck out of me (plus, it had the freeze ray! hahaha). Wrecked had freaky Trainspotting stuff going on, which I loved. Gone cracked me up (flying pizza!). Doublemeat Palace filled me with empathy and made me remember my food service days. I didn't like Dead Things so much, I'll grant you, but I really liked Older and Far Away. I liked how everything with Dawn sort of came to a head in that one. Okay, so I didn't like As You Were (that whole egg thing was never really explained), but Next comes Hell's Bells, which is awesome! Normal Again was a mind trip, Entropy was funny AND we got Tillow back, and then Seeing Red was the most intense episode of TV ever. I think I was shocked at every turn. I was so engaged for this entire run that I just can't help but choose it.
I'll spare you my poorly written litany of the virtues of season 2 and just say that I loved Angelus and I loved Sprusilla. Put the two (or three) together and what do you get? A happy Palabra, that's what! Besides, Giles was so brave.