Your best bets are probably either American or United. They have the 2 strongest alliances by far and will probably cover the most destinations the most conveniently. United and the star alliance is probably the strongest due to problems earning miles with BA on AA, but I personally prefer AA over united. CO and NW probably have less options, but they have lower requirements for their highest level status and a 125% bonus once you achieve it. If you are looking to get the lowest economy class fares perhaps consider Delta. Their partners include Korean and Air France. Korean seems to have the lowest fares consistently to Asia and AF is competitive to Europe. I'm not sure about earning DL status miles with them however.