In all honesty, though, from a pure design perspective, the Gulf livery (especially as executed on the 917K) is perfect. It's simple, pleasing to the eye, accentuates the beautiful curves of the vehicle and it stands out without being offensive to the eye.
That being said, I quite like the Flying Lizard Troy Lee Designs Le Mans scheme, too... which is totally the opposite.
The Renault R26 with the one-off custom scheme for the 2006 British Grand Prix. Absolutely gorgeous. I still maintain this is one of the most beautiful F1 cars ever built.
Honorable mentions to the 1967 Lotus 49 in British Racing Green, the 70s Lotus in JPS black and gold, the 2007-2008 Ferraris bathed in red, and the bright yellow 1999 Jordan with the hornet motife.