The first one. Bruce Campbell, the hero Ash, was probably, like, 20 when he did this one. Ash and his friends go off to an abandoned cabin and encounter an evil force. When it starts off it looks pretty tacky but stay with it. You'll be glad you did. Classic.
My personal fave. This one starts off a bit different than the end of the first one (it's just Ash and his girlfriend Linda that go to the cabin in this one) so if you're a first time viewer of the series you'll be somewhat confused. The eternal question I believe is 'sequel, remake or both?' Me? I just think it's a damn good movie. Bruce Campbell as Ash rocks.
Picks up where ED2 leaves off. Ash finds he's in ancient times fighting off the evil deadites. This is probably the most mainstream of the series. More comedy, less horror, but still good.
The Evil Dead
Evil Dead 2: Dead by Dawn
Army of Darkness
Okay, so this Evil Dead page sucks. Go see some other ones...