One of the funniest costumes I've seen was actually my brother's and his friend's, who both went as the Ambiguously Gay Duo (SNL cartoon superheros). Of course they played the part well too, with the poses and such.

Otherwise, my best costume wasn't all that great, but I took an inflatable pool float's pillow headrest and put it over my head and on my shoulders, then put on a very large flannel over it all to become a headless man. It wasn't the absolutely most creative ever, but the effect was still pulled off well.

Similarly, this year I used cut up kiddie arm floaties under my shirt for exaggerated muscles, got an all black polo shirt with white trim, and latex gloves for Big John McCarthy of UFC. I almost won best costume at the party I was at for it, but got beat out by Superman.