What makes a superb villian, or foil?

Question is the title. This could be a villain in a story someone else made, or one that you've made. Props to the ones that y'all have created!

I realize that for my favorite villians, the reader/watcher gets to witness the full details of just how a villain became a villian. Or, if that process is done off scene, we see very clearly WHY they are who they are. The best villians' demises should pose questions and crises that the reader/viewer themselves either have to wrestle with or at least can palpably relate to from their own experience.

When the process of demise is conveyed, this lets not only the character of the villain be the foil to the hero, but the path itself be the foil to the hero's path. We can 'see ahead' to the kind of crises and dilemmas we know the hero is likely to face, providing the tension and expectation for how, or if, the hero will come through to the other side uncorrupted.