We waited 27 years to learn the story behind the helmet. In Revenge of the Sith we saw Anakin Skywalker lose everything and transform into Lord Vader, the Emperor’s dark side apprentice and right hand man. Darth Vader’s suit has always inspired fear and intrigue but now that we know his story, it makes dressing like him on Halloween even more of a thrill. (Well, for us nerds anyway.) Even if you’re not a dedicated fan, this is the Star Wars costume that will amaze everybody.

By far the coolest and most accurate costume available is the Adult Darth Vader Supreme Edition Costume from Rubies. This costume was created from the actual molds used for Hayden Christensen in Star Wars: Episode III! If you will be attending any Halloween parties with costume contests then this is what to wear to guarantee a win. It’s so detailed and accurate that only super Star Wars nerds will be able to tell the difference between you and the Dark Lord of the Sith.
Includes jumpsuit with quilted faux leather pants and sleeves, flaring cape, gauntlets (gloves), heavy duty injection molded plastic armor pieces (collar, shoulder guards, boot tops, chest piece, belt), supreme edition mask and helmet, and breathing device to mimic the ominous Darth Vader sound of mechanical breathing. The only other parts required are a red Sith lightsaber and a demeanor that says "get me another carmel apple or I’ll force throw you into a galaxy far away". No really, stay in character: it makes a huge difference.
This costume retails for a whopping $999, but you can find it for a lot less online. I’ve seen eBay auctions end for $600 and under and there are plenty of very low starting bid amounts. For a guaranteed price from a reputable costume store, BuyCostumes has the best deal at $799 + $5 shipping.

eBay	BuyCostumes

If your costume budget is less than $100 then you’ll appreciate the smartly priced Adult Star Wars Darth Vader Deluxe Costume. Halloween Jedis won’t be cowering in your wake but you’ll still be a reasonable facsimile of Lord Vader. The main difference between this "Deluxe" costume and the "Economy" costumes is the molded collar, chest, and boot top pieces, which makes a huge difference in realism. The helmet and mask are also of better quality. Pick up a pair of gloves, a lightsaber, and the Darth Vader Breathing Device and you’ll be ready to paint the town red (cuz the Sith use red lightsabers, get it?)
Includes injection molded helmet/mask, flowing cape, belt and jumpsuit with molded EVA collar, boot tops and chest piece.
Shipping prices vary greatly with online stores so be sure to check them before ordering. eBay is still the place to find the cheapest deal, but BuyCostumes has the best guaranteed price of $90 including shipping.

eBay	BuyCostumes
Fright Catalog

Darth Vader may not be the best costume idea for everyone, but it is undoubtedly one of the most impressive costumes available. If you’re looking for a cool addition to a Star Wars themed Halloween party, try to get a Darth Vader cake pan off eBay. They were only released in 1980 and are getting harder to find, but are worth the effort to have a sweet Darth Vader shaped cake!

Other Resources: Official Supreme Edition info, make your own DV costume, TheForce DV costume thread, VaderPainter costuming info