Bernard Lagat, Bill Rodgers, Frank Shorter.
Lagat has been around so long and is so consistent. He is also so kind and charming and classy, you just gotta love the guy.
Bill was a real underdog, seemingly came out of nowhere with his 75 Boston win. Just imagine today someone setting a PR by 10 minutes to win a major city marathon in a new American record? everyone would lose their minds
Frank had an incredibly difficult childhood. But he busted his butt academically and athletically and is the only American to ever medal twice in the Olympic marathon and the only American in over 100 years to win an Olympic gold in the marathon. Then factor in his 4 consecutive Fukuoka wins, the man is a legend. You can argue for days between who is better than who, but Frank and Bill are no doubt the 2 greatest American runners of all time.
Also, respect to athletes that train on their own and are self coached and work hard simply for the love of the sport and to test their limits despite the fact that they'll never have the talent to run sub 2:20 marathon or sub 15 5k or anything like that.