Alberto Salazar was one of the best American distance runners ever. I ran a 5-mile race in Denver, CO where Salazar took the race in 22:05 and my coach mentioned the fact that it looked like Salazar wasn't even trying to push the pace...like it was just a workout run...
As far as the best runner? Would have to give to the below runners in order:
(1) G. Rupp
(2) Frank Shorter ('72 gold medalist in marathon, '76 silver medalist)
(3) J. Ryun (Held HS mile record for 38 years w/a 3:58...Got the silver medal in Mexico City in the mile....Was voted as the Best Athlete of the century by Sports Ilustrated magazine beating out other athletes such as Michael Jordan, Muhammad Ali, etc. (Look it up....)
(4) H. Lindsey (Beat Lasse Viren twice on the roads....broke the 4 min mile several times....Runners World "Runner of the Year" several times between 1979-1984)
5-10: Billy Rodgers, Alberto Salazar, S. Prefontaine, Mark Nenow (Ran a 27:20 10km (AR) back in the late 70s), Steve Scott, Jon Sinclair.