For the non-enthusiast driver (meaning someone who doesn’t drive for the sake of driving or enjoy driving a really good car on a racetrack or Auto-X), a manual transmission is generally more efficient than any automatic because manual gearboxes are low friction and do not have any internal oil pumps or electric power requirements to operate. The actual fuel economy you will get depends heavily on how you shift. You need to understand how to drive efficientyly. (see: How to shift a manual transmission for both performance and economy ( (https://rothautomotivescience.com/how-to-shift-a-manual-transmission-for-both-performance-and-economy/))
Today, many automobiles are rated higher fuel economy with the auto-gearbox because the auto gearbox may have more gears and better gear ratios for highway driving; and because the federal fuel economy test procedure does not allow for the optimal shifting strategy for a manual gearbox. It is easy to beat the rated fuel economy in real-world driving with a manual.
For the enthusiast driver, manuals are favored because you want complete control of when the transmission shifts when you are driving around corners at top-speed. When you are at the limit in the middle of a turn an unplanned automatic shift can really be upsetting to the car and driver! Of course you can get automatics that have manual shift (usually labelled “sport” mode), but these will over-ride your choice of gear in certain conditions.
My personal opinion is that a manual transmission can make driving an otherwise ordinary car, somewhat fun; and an automatic saps a lot of the fun out of driving an otherwise fantastic car.