It is best to buy a water heater from local plumbing shops who stock parts encase the water heater breaks down for some reason.  When you buy a water heater from the home stores, such as Home Depot and Lowe's,  everything must be ordered and shipped, leaving you cold for at least 3 days.  Nathan's Plumbing recommends State Water Heaters and Bradford White Water Heaters due to the extended manufactures
warranty.  Bradford White offers 6 years on the tank and 6 parts.  State offers 6 years on he tank and 2 years on the parts.