Costumes for fancy dress occasions should be more than just off-the-rack purchases. You want an outfit that speaks to you, that reflects something fun and exciting about who you are and that always makes a lasting impression. Also, they should always show you at your very best.
The kind of costumes you have in mind will depend on the theme of the fancy dress get-together, but also on your individual tastes. If it's a Halloween gathering, you may want to consider a wicked witch outfit. For retro parties, maybe a Sixties hippie or...