Bea,
Being a theatre person I have seen many, many a wonderful costume.
Most successful costume at a large party? Waldo. All night people will have a hoot looking for you in the crowd, and you can wear a costume that is comfortable. It's simple enough - knitted hat, red and white striped shirt, Harry Potter plastic glasses, walking stick, etc.
Best costume I ever made for myself? Jesus. Yep. I was Jesus for Halloween at a goth club party. I had a bloody crown of thorns made of dried willows and toothpicks, wrist stigmata action, I gave myself stubble with a stipple make-up sponge, and I wore Birkenstocks and a robe made out of cheap old burlap. It was the best costume because of the reaction I got. Everyone wants to be Jesus' buddy - they buy you drinks, make water-wine jokes, they hug you. It was good. Everyone was saying "Hey Jesus!" all night, and since most people didn't recognise me, even people who I didn't usually get along with wanted to be my buddy. I have a clear memory of standing around with total strangers, someone gleefully exclaiming "I'm smoking weed with Jesus!" Later that night in the bathroom someone exclaimed "Jesus pees sitting down!" Also a goth intentionaly bumped into me, and said something to the effect "Sorry, I couldn't see you, since I don't believe in you." There were two Hunter S. Thompson's - both strangers - that night and each of them helped me become more and more intoxicated. It was a good night all 'round.
As for you, it depends what you're doing - are you going out to a par-tay? What sort of party? I always think function is as important as presentation. You'll never fail if you go as a famous movie fictional character that people have said you resemble. I once was Hester Pryne - I wore a bonnet, black dress and a large scarlet "A" on my chest - a nice comphy costume for the dance floor, easy costume to make, but also a clear and sort of sexy message.
Last year I was Kelly Osbourne, cause in an unrelated to a halloween event, my hairdresser gave me a mohawk a few days prior, and I didn't really have time to shop. Wost people just though I was just being more cool and fashionable than usual, so it was sort of a failure. This year, I don't know.