I have reviewed over the course of time five of the Paizo' Adventure Paths as well as the two APs which were written with an outline and published in
Dungeon. I have played or run as a GM ten of Paizo's Pathfinder APs as well as all three of Paizo's APs published in
Dungeon.
I can (and have!) quibbled about which of the APs is strongest and weakest. There are a number of arguments to be made in favor of several of the APs to be considered among the strongest. There are three which are generally acknowledged to be termed the weakest. Two of those three are referred to above (
Serpent's Skull and
Second Darkness are two of the three weakest APs, with
Legacy of Fire being the third).
However redemptive the earlier adventures are in
Serpent's Skull and
Second Darkness, as you and your players may find that you will wish to continue with the game, I would
strongly recommend that you
not start with either of those APs.
Accepting that it is wise to leave you and your group the flexibility of continuing the AP should you all prefer to do so, the choice rapidly becomes very easy: you should purchase and run
Rise of the Runelords Anniversary Edition.
RotRL is generally acknowledged to be among the best of Paizo's APs. It is the only Pathfinder AP available in one hardbound book. RotRL has been run to completion by literally
thousands of gaming groups so that advice on how to handle so-called "unexpected" developments is readily available to you here and on Paizo's Message Boards.
In addition, the wealth of accessories to aid you in running RotRL, from cards, flip-mats, map packs, AP specific miniatures in both painted and unpainted form, and Paizo's inexpensive "Pawn" line of cardboard standup counters is greater for this AP than for any other specific RPG adventure product, for any game, on Planet Earth.
Best of all, the cost of a six book installment for all of Paizo's APs, even if purchased through Amazon, ranges from about $90 to $120.
Whereas the cost of the hardbound
RotRL Anniversary Edition from Amazon is only $41.58. The best supported AP is also the cheapest.
About two years of gaming goodness for less than $42 is about as good an entertainment deal as ever existed anywhere, at any time.
Rise of the Runelords Anniversary Edition $41.58 (Amazon)