Throughout the years, Ned Flanders is probably the one who appeared on the Halloween specials most, asides from the Simpsons themselves and the aliens, of course.  So I figured he deserved his own little list of what he has been.  Not in the right order though, sorry for that.

The devil
      It's always who you least suspect

A werewolf
      You can't kill the undead, silly

A zombie
      You killed the zombie Flanders - He was a zombie?

A severed head
      cut off by the gremlin: Aww isn't it cute, he's trying to claw my eyes out

Master of the world
      time for a lobotomy!

A Pilgrim
      The witches...Maude, do something

Foreseer
      You'll die eating a Subway Sandwich

Cow-Centaur
      Milk Me!!!

Saviour of the world
      Well my first wish would be to get rid of those nasty aliens (with the monkey paw)

Flower
      I don't want any flowers, I am a flower!