The answer to 'best system upgrade' is that the best HVAC system upgrade for your home depends on a few key factors, including the age and condition of your current system, your home's size and layout, your energy efficiency goals, and your budget. In general, upgrading to a high-efficiency, properly-sized HVAC system can provide significant benefits like improved comfort, lower utility bills, and better indoor air quality.
Some common HVAC system upgrades to consider include replacing an old, inefficient furnace or air conditioner with a new high-SEER or high-AFUE model, adding a heat pump for year-round heating and cooling, or installing a zoned system with programmable thermostats to better control temperatures in different areas of your home. It's best to consult with a licensed HVAC professional who can evaluate your specific needs and recommend the most suitable system upgrade.