Francona and Dick Williams are the clear Top 2 in some order. Francona had more talent than Williams for the most part. But Williams got results in multiple places (San Diego, Oakland) afterward ... he was a turnaround artist with a more limited shelf life than Francona, but he was excellent.
Jimy Williams is probably 3rd (at least from my lifetime of Red Sox managers which goes back to Houk)
Cora is not going to go 144-18. What is interesting so far is the Sox have gotten early improved performance from places where you'd expect performance. The pitching is another thing - unsustainable, but hooray for now.