Jon Tester has the most competitive race. If you are looking to donate to Democratic Senate races, he needs it the most.
Sherrod Brown in Ohio has been doing a bit better in polling than Tester. But state fundamentals are rough, so it may yet be a tough battle. He'll need a lot of split ticket voting and we are seeing that less and less with the Senate. Moreno is a pretty bad opponent, so he may get it.
Next weakest race is Slotkin in Michigan. She's not an incumbent, and non-incumbent polling has been weaker this cycle. But state fundamentals should help her.