Which US treasury security is the most important

2 year and 10 year are most important for the yield curve, roughly representing the short and long ends of the curve. The shape of the curve tells you something, as do the real and nominal rates of the 2 year and 10. (Real rates are rates adjusted for inflation).

The shortest end of the curve is the end that is most controllable by the Fed, so the 1 month T-bill is also important to see what the Fed is directly controlling.