If Whitmer wants it, it is hers at this point. No one would beat Whitmer in a primary election. If she doesn't it will be a free for all.
I think any of the statewide candidates would have the most name ID and could potentially start with the biggest advantages there and in fundraising. So Garlin Gilchrist, Jocelyn Benson, and Dana Nessel would all have the advantage of statewide name ID. Garlin Gilchrist might be the strongest candidate of the three given that he is a candidate of color. This is all depending on if he wants this or to succeed Mike Duggan as Mayor of Detroit. I don't think Mike Duggan will go for it as he wants to be Governor after Whitmer from what I've heard. Dana Nessel doesn't have any real baggage, even though the media and pundits likes to pretend she does and that she's controversial. I think Nessel latest election result over DePerno proved she's plenty electable statewide. Jocelyn Benson was the top vote getter of the major statewide candidates and she is a strong fundraiser. She would be another really strong choice and would play well with white suburban women and college-educated progressives I feel.
Debbie Dingell and Elissa Slotkin will be the two strongest Congressional candidates in the race I think. I might throw Haley Stevens in there as well. Slotkin is probably a better candidate than Dingell, except for the fact that Debbie has the Dingell last name. Rashida Tlaib likely won't run and isn't electable statewide unfortunately due to her religion and the number of Islamophobic people out there, including in her own party. The Oakland County Jewish vote won't go for someone who speaks out against the apartheid state in Israel. As well, Rashida is just too controversial for too many people to win a statewide election. Shri Thanedar is a snake and would once again pretend to be a Democrat and go for it as well. If there are enough strong women in the race, I could see Shri winning that primary on 15% of the vote with all the vote splitting that could end up happening.
Any of the State Legislative candidates simply don't have enough name ID or clout to make the jump to Senator yet. While Mallory McMorrow raised over a $2 million in this last election for herself and the Senate Caucus in Michigan, this primary is going to be a $10 million or more race IMO. Can she get there? Maybe, but there will be other candidates that I think have an easier time doing so.
Garlin Gilchrist, Jocelyn Benson, and Andy Levin would be my personal top three choices for who I'd want to vote for.
My power rankings for the Senate seat would be as follows. These are not my favorite choices, just who I think ranks out as the strongest candidates for a statewide Senate seat.
Gretchen Whitmer (far and away if she wants it)
Garlin Gilchrist
Jocelyn Benson
Elissa Slotkin
Debbie Dingell
Dana Nessel
Haley Stevens
Shri Thaendar (Could win if 3-4 women start splitting votes)
Mayor Pete
Mallory McMorrow