This site has a very good summary. In brief:
The US Army Signal Corps marked its aircraft with only a red star on the tailplane. However, this seems to have been unofficial.
In May 1917, the first official system of marking aircraft was authorised:
The star marking (blue roundel, white 5-pointed star with a red sphere in the centre) on the upper and lower wings. Red, white and blue vertical striping on the rudder, with the blue stripe forward. Nothing on the fuselage.
In January 1918, aircraft in France had their star insignia replaced by a target roundel similar to those of Britain and France, to prevent confusion. This was red outside, then blue, with a white centre.
In February 1918 the tailplane markings were also reversed, so that the red stripe was forward. However, it seems that not all squadrons adopted this rule.
After the war was over, in May 1919, the roundel was dropped and the star re-adopted in its place. At the same time the rudder markings were swapped back so that the blue stripe was forward.
Conclusion? Although a purist might argue for the AEF markings, I think the May 1917 insignia is probably most distinctive for the US aircraft.