Wait a sec... are you asking which one, IF I could get one in select-fire? Or in the semi-auto versions? And are you asking for MY use, or to equip an army, or what I think the US army/marines should use?
Although the answer is likely the same regardless. The 55x Sig is interesting for a variety of reasons; arguably the best choice. The FN SCAR and Steyr AUG have some strong points as well. As does the M16/M4 type.