Sharkeeta gets my vote, too. It's no secret 'round these parts how much I adore her. I've mentioned her a few times to Conrad so here's hoping my planted seeds will bear shark/mermaid/fairy fruit.
Also, she was in George Perez's Through the Years poster, so she's not completely one and done.
A few of my other favorite one off's:
Tigra Tropica, a criminal mastermind who used trained tigers as her henchmen.
Sontag Henya, a Venturian warrior who took over after Queen Clea's imprisonment.
Perfection, a villainess who posed as a meek secretary to uncover the secrets of the Amazons.
Barremargox, a demonic entity and wife to Drax (who is far less interesting visually than her). Cheetah spent some time in Barremargox's dimension and I'm dying to see that explored in some capacity.
Dreadnought, the definition of a himbo. He's a big beefy stud who isn't very bright and wears golden armor shaped like a cat.