single-stage or multi-stage Heatpump
There really is no cure for inadequate duct work that involves making use of the existing ducts.  If one looks at the fine print of super efficient systems, there is always a qualification that the units must be installed correctly to achieve the rated performance.  Good duct work is a big part of correct installation.  If the duct work is marginally bad, adding an inline booster fan to push more airflow where it is needed might work, but that kind of solution reduces both efficiency and reliability.

Ductless mini-splits are sometimes a good option where existing ducts are not usable.  Some mini-splits offer the highest efficiencies available and they also are equipped with inverters that provide variable capacity control that adjusts on the fly.  Also, different models of the indoor portion of mini-splits are available to satisfy different situations.