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I had a K24A2 in my Insight. With custom gearing I could get within spitting distance of 60mpg on the highway. Mind you that's already a slippery car, but the difference between 25mpg and 60mpg is really more in the gearing and less in the engine.
My money is on the L series. They're lighter than D's, and a dime a dozen because they're dead reliable and nobody knows what to do with them, so junkyards are full of healthy L engines. The L15B1 in the 3rd gen Fit makes the same horsepower as the D16Y8. My ex's 5MT Fit would regularly return near 50mpg even with its terribly short gearing. They're definitely more efficient than a K series, and you have more in the way of options and variety than with an R engine.
The 10th gen Civic's L series engine produces around 180hp and has been reported to deliver 60+ mpg if driven carefully.
The advantage of the K series is that there is a ton of aftermarket parts that make any K series engine drop-in for your chassis. You wouldn't be inventing any of it.
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