Trek Y-Foil. Carbon beam frame, aero design, only made for two years. The UCI said it was an 'unfair advantage', whatever that means. The UCI ban killed the Foil, though some triatheletes still favor it.
May not be the absolute best frameset, but IMHO one of the most graceful looking, and at $500-600 for a clean frameset on ebay these days, one of the most cost effective.