Nothing unsingable about the Star Spangled Banner, if pitched right. The tune is a pleasant drinking song with room for enjoyable harmonies. Sounds pretty terrible when sung with excessive emotion. Or "interpreted", which sounds to me a pretty disastrous notion with any anthem.

The fashion for treating anthems as opportunities for hyped-up solo singers to show off is much to be lamented. The thing to do is just play the tune, and leave it up to the crowd or the players or whatever to sing along if they feel up to it.