I'll give an opinion as a non American (hence no emotional bias to anyone/era etc etc).
To me your contenders are the ones laid out by almost everyone here already - Ryun, Centrowitz, Hocker and no disrespect to any of the other great US 1500m runners but they have kind of separated themselves.
Hocker has an Olympic title and the record. I don't know how I feel about Lagat as the national record holder a little under 3 years after he became the Kenyan national record holder but yeah, different topic. I would personally consider Hocker the AR with his run from Paris.
Centro also has his title but he also has 2 WC medals - both behind a guy who was banished from the sport and especially in the case of 2013 (when he won silver) was absolutely doping at the time (discussed his possible doping timeline relative to his performances on another active thread). Not just that, he was relevant the entire 2010's - he made every world final of the 2010's except for the London WC's.
Ryun is mythical. He was so far ahead of his time and is the only WR holder of this trio. His 1500m and mile records lasted 14 years between them (6 for the 1500, 8 for the mile). He has hardware from an Olympics and while it wasn't the ultimate, he lost out to an equally legendary athlete in difficult circumstances (of which we didn't really understand completely at the time) - 7300ft of altitude.
My final wrinkle is that I distinguish between "best" and "greatest". I don't think they are the exact same thing though they are obviously intertwined with each other. I think it is absolutely possible that in a hypothetically era-neutral situation Hocker could beat Ryun, thus making him the better runner. But on the other hand I don't think one epic performance is simply enough for you to be the greatest.
Best US 1500m runner
1) Hocker
2) Ryun
3) Centro
Greatest US 1500m runner
1) Ryun
2) Centro
3) Hocker
The list immediately above is extremely fragile. If Hocker backs that OG win with a WC win he obviously vaults Centro and then it's difficult to separate his global performances with Ryuns records and transcendence of the era. If he comes the pre-eminent championship miler of the 2020's then he will become the greatest 1500m you ever had.