Darth Maul. Perhaps he was the only true iconic Star Wars character to come from the mess that was The Phantom Menace movie. He died a classic death at the hands of Obi Wan Kenobi and thus became a Star Wars legend... which is presumably why Darth is such a popular cosplay character choice!
Here's a few pictures of Darth Maul that I found around the place. You'll see that some of the make up jobs to get the character looking just right are pretty impressive!
Check out the make up job on the above picture, a lot of care went into it - check out the yellow eyes, that's commitment to detail!
Want more? Check out the ultimate Star Wars cosplay page.