|From 2009, the last year I got to go.
Dragon*Con: One of the biggest sci-fi/fantasy/geek conventions in America.
Cosplay is an art form. And if you want to see it at its best, attend a fan convention. Sadly, I missed Dragon*Con this year, but here's a couple of links for this year's Cosplay galleries:
CNET's cosplay gallery
Geeks of Doom
io9's Cosplay gallery
I won't say much else, because seriously, you'll waste enough time admiring the artists' works.