This is insanely difficult, because I really love each season. Season 1 I would rate lowest with a knife at my throat, simply because it doesn't really hook you (me), there are too many one-off episodes (which I understand can be necessary when starting off a new series, you don't wan't to loose viewers who missed one episode. Still, I actually saw season 1 on DVD and then held off buying another season for over a year, can you imagine? I wasn't sure I liked the series that much. But from season two, wow! And returning to season 1 now I really like it.
After that, I saw the rest of the episodes in (very) rapid succession. That could be one reason why I don't have a problem with the more depressing story lines like "Spuffy" in season 6 (if that means what I guess it means). If I have had to wait weeks for her to cheer up and get back in business (hey, you were only dead and brought back to hell, get over it!) I might have felt differently. As it is, I love season 6. And 5,7,2,3 and 4. And 1, when I think about it.