If you want to go to Alaska, and your primary carrier doesn't fly there, then this is where the partnerships come into play. Thus, you should look at all of the airlines within an alliance (American's OneWorld and United's StarAlliance were mentioned before, and are very extensive) and compare the destinations that they serve with your potential destinations. I would also suggest Northwest or Continental (they are partners along with Delta). This means, for example, that you can accumulate miles on NW when flying CO or DL. You can then redeem NW miles for a free ticket on NW/CO/DL.