The two highest rated episodes were S3E22 "Through the Looking Glass" at 9.7 and S4E5 "The Constant" at 9.7.
The two lowest episodes were S2E12 "Fire and Water" at 7.2 and S3E9 "Stranger in a Strange Land" at 7.2.
S6E23 "The End" rated only a 7.9.
The rankings for each season opener:
1: 9.4
2: 9.2
3: 8.7
4: 8.9
5: 8.8
6: 8.8
Throughout the series, the median rating line from start to finish is 8.8 to 8.5.
The first season had a slow decline. The second season started off lower, and slowly went back to the level of the first season. The third season started lower than the second, but quickly moved up to surpass the high of the first season. The next season started at the level of season two and moved up a little. Season 5 started out lower than Season 4 but finished the same. Season 6 started very low and stayed pretty much up and down, with the second half cratering to the end.
