I am a thrift store junkie so there have been a LOT... but right off the top of my head? A Chanel dress for $2.99. Lovely, perfect condition, and not at all dated.
Sold that baby on eBay for $300. Woo!
It's not all about profit though. I bought some pink suede Minnetonka boots last year that still had tags on them ($150) for $5, and they are adorable, and I kept them.