It all really depend on what your going for. If your going for some thing classy, sexy, scary, silly, or fun.
I can give you a list of costumes that can work but it all depends on the person to decide what's the best.
for a women:
You can be Cleopatra or a goddess. A flapper girl or have a really cool retro costume. If you go sexy there are a tons to choose from. Swedish milk maid, little red riding hood, you can find all kinds of different costumes at stores like spenders. For scary you can dress like a terrorist, witch-classic for every year, monster, zombie (might be in this year with zombie movies coming out)
guy:
For silly you can get as original as you want. I personally found the night stand table hilarious. Some guy a few years back made a box as a table. Put a whole in the center so that he could fit in the middle and made it so it stayed. He had a lamp shade on his head to be the lamp in the middle and then on the table he had an empty beer can glued on, as well as a box of condoms, and a bottle of lub... just a bunch of random crap glued on to make it look used. Was pretty fun.
hope this are some great costume ideas that well help you find the right one for you.
Besides once costumes start flying into stores you'll find something that's great for you. But to make it the BEST make it a bit more original and add your own flare. That's how it well stand out and make it better then just a bought costume.







