If you want the pro's answer, here it is (you know, Tim Gunn from Project Runway and Tim Gunn's Guide to Style)?
• Basic black dress
• Trench coat
• Classic dress pants
• Classic white shirt
• Skirt
• Blazer
• Day dress
• Cashmere sweater
• Jeans
• A comfortable alternative to a sweatsuit (usually some sort of casual dress)
http://www.thebudgetbabe.com/archives/220-Tim-Gunns-10-Essentials.html