Best fuel efficiency ever?
 

What aircraft burns/burned the least fuel per passenger-kilometer, or per tonne-kilometer?

To clarify: we need to carry a given payload for a given distance, but we can make as many stops as we like and as many trips as we like. The crew counts as payload; speed doesn't matter. No soaring.

Supposedly the 1-seat Quickie burned around 25 grams per kilometer (and whatever happened to the Quickie?), and a 400-seat 747 must burn about 400 times that much, so there's lots of possibilities. 757-300? A321? A 300-seat Japanese A300? How about an Icelandic 189-seat CL44J? An Airspeed Ambassador? But don't limit it to airliners.

So did they burn... maybe 3000 kilograms? To carry 2 people for 40000 km?

Voyager's flight was of course a unique achievement, but an aircraft designed to fly nonstop around the world won't set records for lowest fuel-burn per kilometer-- at least, not while it's flying around the world.

A winch launch with a high performance twin seated glider (like the www.eta-aircraft.de]eta[/URL]) burns approx .4 kg of fuel and results in about 300 m altitude. This can be used for about 20 km gliding without thermals, so 40 passenger kilometers with .4 kg fuel makes .01 kg per passenger kilometer as worst case.
World record is over 2000 km gliding, if started with a winch launch this makes .0002 kg per passenger kilometer. :D

Assuming we're talking about relatively run of the mill stuff, and not exotic beasts, the two best I know are...

Pegasus CT2K - about 120 kn at about 15 litres /hr
Grob G109b - about 110 kn at about the same consumption.

Both are plastic 2-seaters, although the latter is heavier and has at least some ability to carry a sensible amount of fuel and baggage as well as two adults.

G