While it may carry form person to person, for me the most efficient SAK sone with minimum bulk and maximum tool capability.Or, to phrase of in my my; maximum minimalism.
I'd say about 7 or 8 times out of 10 when I reach for my SAK, its because I need a pocket knife to cut something. Maybe like 80 or 90%. A single digit amount of times, its because I need a screw driver. A single digit amount of times I need a bottle opener/poking tool/light pry tool. And a single digit amount of time I need a scissors. A fraction of one percent of the time I need a can opener.
I've got by without a SAK a lot of times by having a Sear's 4-way keychain screw diver in my wallet. The keychain screw driver got me by as it did for my dad. But a SAK is a nicer way to that end. For me, the most efficient SAK is then and two layer ones like a recruit/cadet/pioneer, and the smaller 58mm like the classic. I've cut stuff with a classic, dealt with flat and phillis screws with a classic, fixed stuff out places with a classic, opened a cold brew with a classic, and its soooo damm tiny it fits on a keyring with room to spare.
The great beauty of the majority of SAK tools is, they are good for uses far more than they were designed for. Like the can opener is a great staple remover, and trailer ball hitch cotter pin remover. It can lift the coffee pot or stew pot off a camp fire. The awl on a pioneer is a great scribe for scratching something like a number onto something, stripping off some insulation from a wire for a electrical repair, making a starter hole for wood screws on a Harry Homeowner shelf project. Each SAK tool has many multi uses.
For me the most efficient are the basic two layer SAK's. Teamng up a pioneer with a classic is a great duo.