Well, that depends. Are you going to have anyone helping you run your effects? I'm usually running the whole show myself, so I find black jeans and boots with a Halloween sweater to be the most practical way to go.
HOWEVER, this year I finally decided to splurge on a really nice costume. After looking through a great many websites and checking out the selection in seasonal stores, I opted for a black velvet gothic ballgown and cloak from Hot Topic. This is a chain of gothic stores with locations in many shopping malls - they also have an ecommerce website. They have several wonderful gowns for about $100 each. This is a clothing, rather than costume, store and the garments are very well made of good quality materials. They also carry a variety of velvet cloaks and some interesting accessories.
If you live in a warm area, Lillian Vernon carries some nice adult costumes (seasonal only) made of light weight materials. These are priced from about $50-100.
X. Some Things to Consider
Before you populate your yard with rubber critters, give some thought to what kind of creatures live in your area. For example, I don't use rubber rats because we occasionally get real rats up out of the sewers if there's been a lot of rain and I don't want anyone siccing the health department on me. I'm sure there are places where large snakes, spiders or cockroaches would merit an equally unfriendly reaction.
All Trick or Treaters are NOT created equal. If anyone mentions a food allergy, take them at their word - some of these can be deadly. Consider having at least one package of non-chocolate, and/or non-nut candy on hand or perhaps some non-candy treats so that all the kiddies can have something.

XI. And Don't Forget:
Have lots of candy! (You'll need it for all the folks who stop to admire your wonderful display!)
Take lots of pictures!