I was looking at a BD-5 that got 32 miles to the gallon for one person and wonder: Which plane gets the most fuel mileage if you divide the number of passengers against the mileage of the plane? Blimps, if flown decisively, can get the most drifting, but for this question I want to narrow the questions to planes and excluding operational costs other than fuel.
There are a number of considerations in answering this question. This will emphasize how aircraft are actually used rather than some best case scenario, as a giant all economy flight might be. The short answer is 787-9 overall with the 321neo making an unexpectedly strong showing.
Widebodies as a class had these characteristics in 2019 as determined by an MIT study: 1.53 departures per day, 265 seats per departure, 12.49 hours airborne per day, 1932 gallons of fuel per hour, and 4,152 nm stage length. Multiplying this out you get 405 seats flying 6352nm using 24,130 gallons of fuel, which is 2.5m seat miles for 24k gallons of fuel or 106.7 seat miles per gallon on average. The 787-9 is arguably the most fuel efficient of widebodies and flies each seat 50km per liter or 117 seat mpg. The giant a380 has older tech and moves each seat 100km per 3 liters of fuel, or 78 seat mpg.
Narrowbody data is more prevalent. I found this which shows more detail:
As expected the class is worse than the widebodies at 80 seat mpg. However, in one case the 321neo at 123 seat mpg (the best of this bunch) beats the 787 average by six seat mpg, but only by considering its best year against all the 787 years. The early years of any aircraft operation generally improve as efficiencies are found, but this is more than one would expect and is likely due to stage length and other operating differences.
A self launching glider like this probably can save lots of energy if it flies the path where it can use its glider capabilities at least time to time. It otherwise has 100 km range done with zero fuel anyway as this aircraft is electric.