As an artist, redlines and paint-overs are ENORMOUSLY helpful. They're obviously quite time-consuming, but every time I've received one I've been absolutely over the moon about it, and it's really helped jump-start improvement in my next few pieces.
Beyond that, concrete examples of what the person giving feedback is trying to describe are great! For instance, if there are issues with movement in a fight scene, linking to a comic page with amazing dynamism and movement in a fight scene, and breaking down what's going on and why it works, will do wonders for me.
I really like feedback that is descriptive, but direct. I don't need sugar-coating. I am going to be stung a little by it, but that's my problem. (ADHD emotional sensitivity to criticism is a... thing...) I'll usually take 24 hours to process it and allow it to settle before looking at it again with more objectivity and less emotion. At that point, I'm able to go through and pick out a few points to focus on improving. Give it a week, and I'm probably taking most of it as gospel and am super grateful for it.