Just to name a few Americans of recent history.
Bryce Hoppel: One of the fiercest competitors I've seen in a long time. Just set a huge AR and looked so disappointed not to medal, even though he clearly pushed himself to 100%. I remember in a race in Budapest he followed the leader out in 48.X and then kept pushing and pushing even while tying up and afterwards was lying on the track breathing so hard I thought he needed to be defibrillated.
Connor Mantz: I love how much he races, even on the track, as a marathoner. Last year he jumped in a 1500m and finished in 3:37, off marathon training. If there's a race, anywhere, anytime, any distance, you can expect Mantz will show up and give 100% and place at least decently. He is the opposite of Mu and McLaughlin.
Bernard Lagat: We're talking about a dude who at age 41 won the U.S. Olympic Trials 5000m with a monster kick, smoother form than all the 20-somethings he blew by. The great Lord Coe would always comment on how Lagat's stride in his late 30s and early 40s looked "majestic." If Lagat at age 37 and current Cole Hocker were coming into the final 200m of a slow race, I'd bet on Lagat.