Plenty of vintners dream of making the best Cabernet in Napa Valley. It's an elusive achievement. There are perhaps 800 individual bottlings of Napa's signature wine, probably more; nobody knows for sure. But there's no doubt about this: Napa Valley Cabernet is as competitive an environment as exists in wine today. Few have what it takes to make it to the top. Fred Schrader has made it twice. Wine Spectator senior editor James Laube explains.