Grey Area! Grey Area!
Well...how do you define power? Clearly, curses hold much power in Arda. From the curse of the Noldor, to the curse of Morgoth, even to the curse Isildur places upon the dead men under the mountain. One man (granted he's a decentant of Elros and of royal numenorian descent) placing a curse on a group of people, granting them an immortality until they fufil their oath. That seems like a powerful one as well! Morgoth's curse upon Hurin clearly is the more tragic. I don't think any of us say "Wow! I wish I could have been Turin! His life was fun!" The fact that the curse of Melkor rules the entire lives of the children of Hurin is rather interesting. I'm still getting more and more intrigued by Isildur's curse. One man casting a curse more powerful (well, that's what this thread is about, isn't it?) than that of the valar. The valar's curse works on a more massive scale, and if you measure power in that sense, the answer would be the valar's curse. Making many suffer for that which they may not have deserved (not counting the dead). I'm not really sure what to say, because A) I'm not that familiar with the curse of the valar, and cannot find my copy of the sil, and B) Because I'm not sure how to measure power. Because of my lack of knowlege, I'll side with the Valar's curse on the Noldor. It's on a more widespread scale, and the wrath is spread more rashly
3000th post! Wowzers!