Kelly, Brshear and Shapiro would be terrific choices, certainly from the standpoint of being able to defeat Trump, IMO. However, I also think @Austinfan is right that the clear, and perhaps prohibitive, favorite is Harris.
From Trump's perspective, the best choice would be Harris. His arguments about the alleged failures of the Biden administration can be directly tied to Harris, including of course the signature issue of border security.
Picking someone other than Harris, I think, best allows the Dems to paint the race as an ideological battle, if that's what they want to do. The problem is that any prospective Dem alternative has a political future to think of, and either a failed (a) challenge to Harris as the nominee, or (b) loss to Trump would likely cost him or her that future.
There is one name to which that risk does not apply, and which is in keeping with the kayfabe: The Big Rematch and Cage Battle to Save Democracy
Hillary Clinton.