Like Mr. Mike, I have maintained a spreadsheet with 0-4 star ratings for each episodes, and then an overall rating average for each season. I’ve found that I sometimes adjust my ratings as time goes by and I re-watch the episodes. Here’s my current rankings (with the average rating on the 4-star scale):

1. Season 3 (3.44)
2. Season 6 (3.21)
3. Season 9 (3.20)
4. Season 2 (3.16)
5. Season 4 (3.15)
6. Season 5 (3.08)
7. Season 7 (3.04)
8. Season 1 (3.01)
9. Season 8 (3.00)
10. Season 11 (2.22)
11. Season 10 (2.21)
12. Season 12 (1.76)