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I'd vote Second Darkness ... for a few reasons.
1) The background narrative is integral to the history of Golarion on a more sweeping scale than Curse of the Crimson Throne
2) Its potential was not realized, and that's something a revision could correct.
3) As noted, the support products for it could use a re-vamp. While Into the Darklands and The Great Beyond have not been altered much by the passage of time, additional looks would not hurt. I think all of the relevant stat-blocks from those have been updated by now, so new creatures would be welcome. Elves of Golarion, on the other hand, was done for 3.5 and in the transition period in which Paizo was trying not to put new mechanics in sourcebooks. Some details of the Elves need to be corrected (sleeping vs meditating, for one), and the various Ethnicities need to be defined. We have the Snowcasters, but we only have the Half-Elf versions of the Ekujae and the Spire Elves.