Ok, so not really sure how we are putting Chelimo in this discussion, but not Lagat. If we are talking people during their time representing the USA regardless of birth Lagat is way better.
Current List: (2 of each Gender to keep it balanced)
Bernard Lagat: US #1 All-time in 1500, 3k, and 5k, 5 WC medals to Centro's 2 (Gold and Bronze in 1500, Gold and 2 Silvers in 5k, compared to Centro's Bronze and Silver at WC). While Centro has more Oly success for the US, keep in mind Lagat has even more medals from representing Kenya. Also, he is definitely the best Masters, He ran a 62 min half at 43!
Galen Rupp: Obviously the best American born distance runner
Jenny Simpson: US All-time #2 in 3k and #3 in 1500, 3 WC Medals (1 Gold and 2 Silvers in 1500), and Olympic Bronze in 15
Shalane Flanagan: best range from 5k-Marathon with close times to Huddle, but more big places, also Coburn is better at her event, but lacks range.
Possible Replacements in the Future:
Paul Chelimo: Has a chance of matching Lagat's times and being competitive in international competition, We will see if he has the longevity, currently he is on par with Leo Manzano's accomplishments
Jordan Hasay: Lacks range due to minimal success on the track, but should be able to surpass Kastor for the US Marathon Record (Ran 2:20.57 to Kastor's 2:19.36 in only her second race at the distance), keep in mind Kastor won Oly Bronze, and Shalane only has one medal.
Matt Centrowitz: Same as Chelimo just a few years older, hasn't matched Lagat and is barely ahead of Manzano, Andrew Wheating, and Alan Webb over 1500