The best brand is your local contractor! When the “highest rated” furnace heat exchanger is improperly installed without matching the airflow to the heat, then the metal will overheat and melt or “burn off” the protected layer installed on it. It will take a few years for these “hotspots” to become obvious. Remember that a heat exchanger contains the gas exhaust (which could contain Carbon Monoxide) and prevents it from blowing into your home.