I've been looking into these, just for kicks, and I was wondering: Which do you folks think is the best?
We're in the middle of Curse of the Crimson Throne right now and it is one serious tits machine of a campaign.
The combats are fun and challenging, but the role-playing scenarios they have as a backdrop is really engaging. As the adventure path progresses we have found ourselves not making black and white ethical choices, instead it has been all shades of grey. There are moments when the characters are actually making deals with the devil in order to try and achieve the most good for the city of Korvosa. The adventure path is neither simple nor easy even at its low level start.
So far I have no choice but to give Curse of the Crimson Throne two thumbs up.
EDIT: Actually, come to think of it, flyingmice would probably really dig it because it is a great example of mature adult themes in an RPG adventure that are exceptionally well done.
There are all very good. My favorite so far is Second Darkness. If you've ever played Baldur's Gate II, you are going to love this campaign.
The current one, hands down. Legacy of Fire starts with a super module by Erik Mona himself, spanning 5 levels of play - Howl of the Carrion King. That module sets new standards regarding what can be accomplished in Paizo modules. No railroading, no Jiminy Crickets NPCs who meet the PCs at (or near) the module's entry point and lead them by their nose through the entirety of the module. (If, on the other hand, you like such things, Second Darkness delivers. Big time.) It's a bit hard to believe that Mona would pull such a feat after having contributed to the not so impressive Expedition to the Ruins of Greyhawk. But he does, with a vengeance. It's the first 3.5 module in many years which leads the party into a TPK trap if they don't play skillfully.
So I'd recommend you to purchase just that one module and try it out. If your group likes it, I'd recommend purchasing the two follow up modules in the series and then buy Necromancer's City of Brass boxed set. That combo should deliver a once in a lifetime campaign experience.