Alright, these seem to be the thoughts so far:
1. Undead are a type of Demonic Parasite that inhabits once living beings, suffusing them with Negative Energy and displacing their soul. The Buffyverse theory. The soul of the being that previously inhabited the body is now free, and probably in the Outer Planes somewhere.
2. Undead are another being of some kind, but in this theory, they are not Demonic, and the soul of the previous being is either trapped in the body (the 1E life force theory) or tormented by the knowledge of what has happened to them/their body.
3. Undead are the same being they always were, except that they now use negative energy to sustain their existence, instead of positive energy. If the Undead is ever killed, the soul is sent to the Negative Energy plane where a very nasty fate awaits them. You don't wanna be turned into Undead in a world where this theory is the law. Sigils Theory.
4. The Mind-Body-Spirit are all interconnected, and the perversion or removal of one or more of these three things is what causes Undead to exist. The imprint of memories exists in bodies, and while the soul/spirit no longer is present in the Undead, it still retains all it's knowledge and skills because the body itself retains those memories. This seems to originate from the Egyptian Afterlife theory, so I'll call it that.
It's pretty much agreed non sentient Undead do not retain the soul/spirit of the being they previously were, so let's lay that issue to rest. Although, it would be good to remember that in some theories, the spirit/soul of the beings body who was reanimated would be tormented by the knowledge of what had happened. This would give credence to why the Animate Dead spells are considered evil, as well.
I'll give my opinion on all the theories as soon as I have some time, and if I missed any theories, feel free to point them out. This is great stuff guys, thanks for giving me all these ideas.
