I actually wrote a paper for some friends a while back.
On the original "Evil dead," I vouch for the standard THX release from Anchor Bay. It has more features than the Elite edition. And even though it has less features than the Book of the Dead Edition, it costs way less and the case is far more practical (the Book of the Dead case looks cool, but is out of place on a shelf). The only downfall to both of Anchor Bay's latest releases are that they are cropped to be widescreen, as opposed to the original full-frame.
For "Evil Dead II", Anchor Bay's THX release (sometimes called the Special Edition) is the only choice. It's a cleaner print and has more features than the previous edition. Just avoid the Limited Edition of the DVD--it's pretty useless to pay extra for a tin case.
Now, "Army of Darkness" is a little tricky. The best versions are from Anchor Bay. Their theatrical cut is a better print and has more features than Universal's release. Anchor Bay's other release, now called The Bootleg Edition, has good features and a longer version, but it is not a Director's Cut. It is a cut poorly pasted together from various international prints--it's missing footage, uses alternate dialogue the director didn't like, and has poor picture quality. But, like I said, it is longer and interesting to watch. I would purchase both, but if you're only going to get one, stick with the theatrical cut.
