Regarding 'is Kennedy the best track runner (>1500) ever in the US?' Given that a primary argument is that he is the best runner of the modern generation and runners from the past can't compare, I would suggest that Henry Marsh is a better modern runner (or at least highly comparable).
Much of the perception of Kennedy's greatness is based on two facts; 1) Kennedy and Williams dominating the American 5k and 10k competition in one of the weakest eras ever for Americans, and 2) the 12:58. 13:00 is such a mythical barrier and Kennedy was the first. It is just arbitrary where their PRs happened to land; if Moorcroft had run 12:59, Kennedy's run loses some significance.
Kennedy was only world-ranked twice ever. Marsh was world-ranked 12 consecutive years and three of those were #1. Both have a long-standing American record and never medaled. Kennedy was a virtual non-factor when competing against the world's elite, Marsh was not.
There is the argument that the steeple is a weaker event, but if that is so why aren't people (Broe, Dobson, etc.) focusing on steeple to pick up an 'easy' American record and championship medal?