I think the best ones I've seen have generally been secondary or obscure characters, instead of mainstream. We did Toy Story one year, and I swear I was the 473rd Jessie there that night. But my mom and dad were Mr. and Mrs. Potato Head, and they got tons of compliments!
Here are some of my favorites that I've seen in the last two years I've been:
Flora, Fauna and Meriweather
Sorcerer Mickey and brooms
Gaston and the three silly girls (this one got picked to ride in the parade)
Haunted Mansion CMs (sort of like french maid and butler, but with green and black, like the CM costumes on the ride)
Mary Poppins and Penguins
Dr. Facilier (Shadowman from Princess and the Frog)
Carl and Russell from UP
Random characters on the space ship from WALL-E (wearing padded red jumpsuits, carrying giant soda cups)
We decided not to do the party this year, and I'm bummed because I had a great costume idea! I wanted DH and I to be Gary and Mary from the Muppets! Complete with Walter doll