I want Hinterlands to be the free no matter what set. If only for the hipster factor.
Got to be the number of beards/mustaches in the set.
By my count: Develop, Scheme, JoaT, Nomad Camp, Silk Road, Spice Merchant, Cartographer, Embassy, Haggler, Margrave, Stables (11).
Other sets:
Base: Woodcutter, Bureaucrat, Feast, Militia, Smithy, Market, Mine (7).
Intrigue: Pawn, Swindler, Baron, Conspirator, Mining Village, Duke, Minion, Saboteur, Tribute (9).
Seaside: Ambassador, Smugglers, Bazaar, Tactician (4).
Prosperity: Loan, Counting House, Mint, Mountebank, Rabble, Venture, Goons, Bank, King's Court (9).
Dark Ages: Beggar, Hermit, Sage, Ironmonger, Marauder, Procession, Wandering Minstrel, Band of Misfits, Bandit Camp, Graverobber, Junk Dealer, Pillage, Rogue, Madman, Mercenary, Sir Martin (16).
Well Dark Ages blew that theory out of the water. Not even going to look at the other sets...
On another note, I foresee a "Best Beard Art Bracket" being a thing in the future.