I would say it depends on if you want to make, buy, borrow, rent and your commitment to any of those options.
If you wore any costume from these big movies out currently...Xmen, Transformers, Ironman, Pirates of the Carribean...the kids would go NUTS!
BUT..
those would be costumes you would have to buy from someone here or make yourself using one of the fine tutorials featured on this site.
As for just buying (cheaper kinds of costumes), renting or borrowing...there's always pirate, MAYBE clown, or cute animal characters
depends on age group and if it's for boys or girls...not to mention your own gender