If it is really down to Warren, Klobachar, Harris and Gretchen Whitmer then it seems like a pretty strong group.
I think any one of them could be a great VP. They have vastly different strengths and weaknesses both experience wise and politically though.
If you want a VP pick to help in the general election Whitmer could help filp Michigan back, and Klobachar might appeal across the board really well. That might be the most important thing, I dunno. Warren might (but might not) help bring in the Bernie people to not stay on the sidelines. But lots of middle of the road voters might not want Warren. I have been impressed by Whitmer's press conferences, she seems intelligent and thoughtful (I know that is no way to judge her potential ability as a VP, but that is all the info I have on her)
If you are picking someone to step in if something bad happened to Joe, I think it is a different choice. If I had to choose who I think is best prepared to be President, I would say Warren. I think she has the best combination of knowledge, poise, empathy, experience, toughness and best handle on policy to be able to step in if something terrible happens.
I was honestly surprised when Biden announced he was selecting a female for VP so early in the process. But there are so many qualified women, and his best options all seem like women anyway, so maybe it wasn't even that big of a move to make.