They're all vastly different from eachother.
Evil Dead one has very little of the flamboiant humor that runs rampant throughout the second and third one, but it is the most suspensful and creepy. It has some great camera work and a damn fine atmosphere.
Evil Dead 2 has alot more humor, it's basically a remake of the original with some wackiness....a lot of wackiness. This is usually peoples favorite because it combines the best aspects of the first and third one. It's still somewhat creepy while having some great laughs. This is the movie that we see Ash begin to turn into the god of asskickery as he's known today.
Army of Darkness is a comedy, no suspense, no scare tactics, it's just a wacky adventure movie. This one is probably my favorite, it's the most entertaining as far as I'm concerned.