It was Moder for me, a frozen hell. I won on the third try.
Mostly because of the combination of factors.
1. the terrain, a cliff on top of a mountain, really small to fight a dragon, and taking the eggs to the top was a nightmare.
2. How long it was, it took me almost three days ingame to kill her, my weapons broke and I got out of arrows in my previous attempts.
3. Plus, at night, drakes, wolfs and a golem spawned just next to the cliff and the space was so small that it was impossible to avoid them.
4. Since I made sure to be super prepared to fight every boss before Moder, I think I just underestimate her and didn't bring enough potions and weapons.