When ranking their APs, the best are far and away the early ones, and they released some stinkers in the middle of PF1.
There's no question that they started off hot, with two of their most popular APs. But whether there was a general downward trend in quality is an interesting question.
Looking through the 25 APs that Paizo has released so far, the APs I feel were generally not well-received were:
Second Darkness (3rd)
Council of Thieves (5th)
Serpent's Skull (7th)
Wrath of the Righteous (13th)
Mummy's Mask (14th)
Giantslayer (16th)
Hell's Vengeance (18th)
And the APs that I felt people were pretty enthusiastic about were:
Rise of the Runelords (1st)
Curse of the Crimson Throne (2nd)
Kingmaker (6th)
Carrion Crown (8th)
Skull and Shackles (10th)
Reign of Winter (12th)
Iron Gods (15th)
Hell's Rebels (17th)
Strange Aeons (19th)
Ironfang Invasion (20th)
War of the Crown (22nd)
So while I share the gut-instinct reaction that their first APs were the best, there doesn't seem to be a clear correlation between when they've been put out and how well received they were.
I'm inclined to think my feeling that the first APs were the best ones is at least partially a result of "AP-fatigue". I mean, no matter how good your APs generally are, it's hard to keep up when so many keep coming out.
I'm a good example of this -- I'm generally a fan of Paizo's APs, and followed them pretty carefully, but I stopped reading through them (in part due to life and being busy, but in part due to fatigue) starting with Hell's Vengeance (18th). And I feel this was true for a lot of the PF community.
I mean, I don't even know how well the last two "big blockbuster" PF1 APs were received (the 23rd and 24th), because no one was talking about them. They might have been fantastic. But everyone was just talking about the PF2 playtest...