GP4000 is great but expensive and the threads, stiching are on the tread side rather than the base tape side. This means-you can't repair and it doesn't 'stick' as well to some deep rims where the stiching tends to conform to the rim better.
Most supple are Vittoria CX, Challenge and Velomax. More durable are Conti Sprinter. I think the Sprinter is the best all around tubular.