Could you provide any more info on how this test works? Where are the tests performed from, or do they run client side? Does it query name servers directly, or if not, how does it avoid ISP-level DNS caching influencing the results?
Actually the solution you're suggesting is one of my pet hates. An alphabetical list of countries should be alphabetical! If you put some countries out of order you just make it more difficult for people to find their country if theirs isn't the one pre-selected.
If your audience is mainly US, then by all means pre-select the US, but keep the list alphabetical. If your audience isn't mainly in one country, either don't select anything, or for extra bonus points, do a Geo IP lookup and pre-select the country they're in.