I recently bought the app version of Elder Signs.

Which of the app expansions is the best value?

This board is used for the boardgame version of the game; the board to post questions about the app is this one here

(sayin' because on the right board you may obtain more answers)

As for your query: depends on what you want. All the three campaign are good, and all the three campaign have more or less the same length. Ithaqua's the hardest one, no doubt; I'm really fond of Cthulhu; Nyarly has a nice theme but for some reasons it never really clicked on me