If efficiency for an internal combustion engine equates to the most energy extracted from the fuel with a minimum going to waste as heat, etc, for a petrol engine the BMW B48 engine is up there. Getting that efficiency costs money though, spent on things like variable valve lift and twin spool turbocharging. I’m not sure if any other mainstream manufacturers go to the trouble.
From the user point of view, looking at the car as a whole it’s a question of matching the power output with the vehicles weight and drag characterists and how it’s going to be used. A hybrid will work well for local low speed journeys but not so much bashing up and down the motorway. And then the most efficient car for your particular use may not suit you from other aspects, like comfort, handling, noise. There is no simple answer…