At its base MadOxMalachi has it well put, the villain must be in conflict with the protagonist. From there it depends on what or who the villain is. One type is the main villain is a big bad guy deal, Sauron or Emperor Palpatine to name two of the most classic. Both of these villains are the leaders of big bad empire's which are villains on their own, but in both cases they are the head of the metaphorical snake that is the whole villain to the heroes of their stories.
To go to what Vio said, it really depends on what kind of story you want to tell. If you're telling a story about a sailor stranded on an island then the main villain will be the island itself, and perhaps once the hero conquers that, the villain becomes a part of the sailor battling to remain sane by himself while waiting for rescue.
Going to Eukocar's example, the common soldier from an axis country during World War 2 can fit the criteria. You can take that in many different directions. One seemingly random enemy soldier who kills the protagonist's buddy can become the target of the protagonist's lust for revenge, as he hunts the guy down. I personally don't like throwing the common trooper under the bus without provocation, very often the real villain in war, is the war itself, or the causes behind it, politics, religion etc.
Building on what Rance said, villains who are 'bad' for a reason are some of the best. He also has a big point that the family member that is sick or dying being the cause of the creation of the villain is overused, but if done in a creative way I believe it can be quite enjoyable.
No matter what as the writer you must know why your villain is doing what they're doing, and you must want them to succeed as much as they want to, or else the villain will feel weak and flat. A good way to think about it is to look through the eyes of your antagonist and see that from their perspective
they are the hero, and they will stop at nothing to attain victory.
In my own main story, I'm covering a Civil War, and I root for the Merchant Republican Rebels just as much as I root for the Imperial Loyalists. I have my preference of course because I am the god of that world and I know all of the elements that caused the bloodshed but that doesn't stop me from wanting both sides to win equally, they both have valid points. That is what I think makes one of the best antagonists, is when they have a valid point they are trying to make. They may just be going about it the wrong way.
Alright, I've rambled more than long enough, I hope this helps you out and I look forward to seeing what you come up with.
