What is the most energy efficient commercial HVAC system available?

If you do a google search with those search terms, you won’t get any exact hits. The hits you get will be regarding residential systems, which are much simpler. For residential applications, only the efficiency of the equipment itself (condensing unit and matching indoor A coil) is considered, not system efficiency or the combined efficiency of the components factoring in operating times and percent loads. The EPA concluded in 1993, and it’s still true today, that ground source heat pumps are the most efficient residential system available and they likely are one of the most efficient system for commercial buildings where they can be employed (enough land for the well field). For commercial buildings, there are many more considerations. The ASHRAE advanced energy guides (found here) are the best tool I’ve found to make sure that you’ve considered the best options for your building.  For commercial buildings, the load profile, availability of utilities and maintenance requirements must be considered, among other things.  Also, architectural decisions such as the amount, type and orientation of the windows and building potential for natural ventilation factor into energy usage and potentially, HVAC system selection.  So the short answer is that there is no short answer.  However, some of the most energy efficient systems for commercial buildings include a VSD equipped water cooled chiller paired with a cooling tower and free cooling, a heat recovery chiller if the building has substantial year-round cooling needs, ground source heat pumps, and VRV systems.