My choice would be Gavin Newsom, but I have to qualify that choice. His viability as a candidate would depend on the number and severity of skeletons in his closet and how the MAGA supporters would magnify them to suit their purpose. Newsom is young, feisty, has a great memory and debating skills, and can dish out as much as he gets. I don't think that Trump would stand a chance against him in knowledge and intelligence, and Newsom would roll all over him in a debate. The weak points for Newsom would be the potential skeletons in his closet (I have no idea of what might be there), and the fact that he comes from California, a state which is already voting Democratic...It would be much better if he were from a swing state instead in order to pull independent voters into the Democratic camp. Also, remember that California has the fifth largest economy in the world, so he already has enough experience to run a country IMHO.
Kamala would be my second choice, as she also has youth, intelligence, feistiness, and many of the same qualities as Newsom. Also, with so many issues affecting women in this election, she would win over a big portion of the of women voters, both old and young. She has also most likely been well vetted, so there will be minimal problems there, but she suffers the same "weakness" that Newsom suffers from in that she is from California, a state that Democrats already have in their pocket. When Hilary ran in 2016, America wasn't quite ready for a woman president, a factor that helped get Trump elected, but I really think that NOW is the right time. And let's not forget that she has the incumbency factor on her side also, since she is currently serving as VP.
So Newsom is my pick, but Harris might be the better choice in this particular election cycle, especially considering the short time that any new campaign has before the election takes place.