Graham Williams era for me, but not immediately.
I'm fine with most of S15. For the most part, the tone has't got too silly yet. Fang Rock and Fendahl are particular highlights. The Invisible Enemy is still good. Apart from the introduction of the tin dog, it's no worse than some of the lesser stories of the Hinchliffe era. Similarly, The Sunmakers may not be Holmes' greatest story, but again it's still fine. I still class it as a good story.
Obviously Underworld is dreadfully dull and probably the weakest of the season, but it's not as though there haven't been dull stories before, so I don't see this as something endemic to the Williams era alone.
Which leaves us with The Invasion of Time, which I always find such a let down and a bigger sign of worse to come. I'll save a proper critique of the story, because this discussion is kind of prompting me to re-evaluate it, as it's been a long time. But were it not for Underworld being particularly boring, then I'd have no hesitation in nominating it worst of the season.
My rankings and scores for S15:
Image of the Fendhal - 8/10
Horror of Fang Rock - 8/10
The Sunmakers - 7/10
The Invisible Enemy - 7/10
The Invasion of Time - 5/10
Underworld - 4/10
Total season score: 65%
Whilst S15 was a sign of things to come, the rot really sets in when Douglas Adams' influence further undermines the overall tone of the program, and combined with Tom's ego starting to go off the rails, things start to err slightly more towards the asinine.
Even so, I can cope with most of the Season, except for The Pirate Planet, although it does have that beautiful fleck of gold in the turd, with Tom's: "WHAT'S IT FOR!!" moment, showing he could still produce great moments of drama when the script allowed for it.
For me, Stones of Blood' is by far the best of the season, capturing the essence of the Hinchcliffe era (for 2 episodes at least) one final time.
The rest of the season is not so much bad, as just average, with nothing else really standing out. I think Power of Kroll perhaps just about has enough going for it to scrape an above average score and second place. Leaving Androids of Tara to limp ahead of Armageddon Factor on the basis that at least it's not long. Shame that Holmes' only other script (Ribos) only really shines in Ep.1.
My rankings and scores for S16:
Stones of Blood - 7/10
Power of Kroll - 6/10
Androids of Tara - 5/10
Armageddon Factor - 5/10
The Ribos Operation - 4/10
The Pirate Planet - 3/10
Total season score: 55%
Season 17 is where things start to go seriously downhill. The level of clowning from in S17 (and 18) ia at its height, (cue: the inspiration for Tennant) and the introduction of Romana II brings a level of smug superiority between the two leads that had never been seen before. And it would take NuWho to escalate it to level beyond that as we see with Tennant and Piper.
To be fair to Doug Adams, his stories aren't worst of the Season, but the overall level of quality is another drop down from S16, as S16 was a drop from S15, and in turn S15 was a drop from S14.
My rankings and scores for S17:
Destiny of the Daleks - 6/10
Shada - 5/10
City of Death - 5/10
Horns of Nimon - 4/10
Creature from the Pit - 4/10
Nightmare of Eden - 3/10
Total season score: 38%
And I know it's not a Williams season, but just for good measure:
My rankings and scores for S18:
State of Decay - 6/10
Keeper of Traken - 5/10
The Leisure Hive - 5/10
Full Circle - 4/10
Meglos - 3/10
Warrior's Gate - 2/10
Logopolis - 1/10
Total season score: 37%