21. Donna Martin (Tori Spelling) as a Mermaid in Beverly Hills, 90210
Air Date: 10/31/91
Besides being hot, we have to give props to Donna's mermaid costume for its impressive detail. Of course, she spends the entire Halloween party bumbling around, unable to dance or use the bathroom because of her tail and fins, so maybe there is such a thing as "too thorough."