I've tried a lot of different systems and I think my 510 cartomizers are the most efficient, but you don't get something for nothing. The tanks make considerably more vapor along with other systems like the Clearomizers so naturally, they consume juice faster. There's also voltage and resistance to consider. The more power you vape with (lower resistance or higher voltage), the more juice you're going to use simply because you're making more vapor. Probably direct dripping is the least efficient system only because some amount of juice can be wasted by overflow. However, a drip shield can resolve that problem easily enough. I'd say efficiency boils down to the amount of vapor you're producing. If you select a lower producing system, like a standard 3 ohm cartomizer, you'll get better mileage off your juice. But, you sacrifice vapor production. Something like a LR tank atty is going to put out a lot more vapor, but it's going to go through juice a lot faster.
Mixing your own juice can be much cheaper. There can be a huge difference there, especially at lower nic levels. You can save even more when buying dilutants in bulk. For anyone looking to stretch their vaping dollar, mixing your own is the first thing I'd look into.