The question "what is the greatest Champagne house?" can give you a wide range of answers. One that you will hear quite often is "Krug". Its not a large house, but Krug is respected the world over. Krug Champagnes are mature, full-bodied and, unlike most other Champagnes, are not ideal apertifs. They should be enjoyed with meals.
Krug Champagnes tend to age very well (even for decades!) and the quality is remarkably consistent. This is due to the quality of the vineyards used (Krug owns some of the greatest vineyards in the Champagne region) and the traditional production methods they still employ.
If you want to taste perfection in Champagne, you should try Krug at least once in your life.