Well, lacking Iron means no Swordsmen, no Knights and no Railroads. That means you're crippled from late ancient times until the end of the game. So, I guess I'd vote for Iron.
Perhaps more interesting and answerable question: which is the least important resource? My vote, I guess, goes for salpeter - you can't wage offensive war efficiently in the late-medieval to late industrial times, due to no Cav, but Musketmen aren't that much harder than Pikemen (or Samurai, if you happen to play Japan!). Other opinions?