The best placement for HVAC systems depends on the type of system, the size of the space, and the desired air flow. Generally, ceiling-mounted systems are best for larger spaces, as they can provide more even air distribution. Floor-mounted systems are better for smaller spaces, as they can be more easily adjusted to provide targeted air flow. Ultimately, the best placement for HVAC systems should be determined by a professional HVAC technician, who can assess the space and determine the most efficient and effective placement for the system.