I think she's the most endangered. Hard to see her hanging on again unless it's a good Democratic year, or Republicans nominate a weak candidate.
What makes Donnelly less endangered? I'm aware he survived 2010, but his opponent was Walorski - not exactly the strongest specimen.
Donnelly has charted a more moderate path in the Senate than McCaskill and was part of the Blue Dogs while in the House. He also has stayed clear of making incendiary SJW-esque comments unlike McCaskill and is from a state (Indiana) that is probably a little less red than Missouri.
Claire McCaskill is also more vulnerable than Heidi Heitkamp for that reason as well.
Honestly, I have to agree that I don't see much of a way forward for her other than a Democratic wave or a really poor Republican opponent. I really think that "shut the hell up" comment is probably the last nail in the coffin for her otherwise.