My mate and I forgot to get a costume years ago and so we just turned up as Zorro and Zorro's Dad. It involved tying a bit of black fabric around your face with 2 holes in it, wearing lots of black, a white shirt and a bandana (one with grey hair poking out the sides). We only did it because it utilised things we had lying around but it was so popular, we just wore it to every subsequent fancy dress party no matter what the theme. Probably the most successful outfit we've ever worn as he pulled EVERYTIME when he wore it (I already had a ladyfriend).

Also, I know that the best ones are often dependent on a theme, but if you could to a fancy dress party as ANYTHING, what would it be?

I fancy making this (the guy on the right):
http://www.homotron.net/images/homotron/awesom-o.jpg

Yeah, I know. Fancy dress sucks etc etc (NOTE: It does if people take it seriously and dress up in rubber superhero outfits, if you make a dick of yourself it's great fun).