The answer to 'best system upgrade' is upgrading to a high-efficiency HVAC system. Newer HVAC systems are much more energy-efficient than older models, which can save you a significant amount on your utility bills. These systems also tend to be more reliable and require less maintenance over time.
When considering an HVAC system upgrade, look for models with a high SEER (Seasonal Energy Efficiency Ratio) rating, which indicates the system's cooling efficiency. You'll also want to choose a system with the right capacity for your home's size to ensure proper heating and cooling. An experienced HVAC technician can help you select the best system for your needs and budget.