Just to get discussion going...
Not considering the purchase price of the aircraft, what would be the most cost effective aircraft.
In other words: which Single engine aircraft has the lowest price per NM (on average without wind)?
Would it be a slow Touring Motor Glider of would it be a fast TBM850?
Just to get it started:
C172 EUR 150 per hr at 100kts = EUR 1,50 per nM PA32 EUR 300 per hr at 150kts = EUR 2,00 per nM
The C172 carries max 4 people (38 cents per person per nM), while the PA32 max 6 people (33 cents per person per nM)
If you want to calculate cost per person and always assume the airplane will be full, the answer will most likely be the largest SE aircraft. That should be a PC12 or Cessna Caravan or maybe AN-2.
A 4 seat motor glider, flying close to Vbg, should be the best 
Other than that, physics will always rule in the end so most petrol powered retractable 4-seaters will be within a fairly narrow band, especially if you set some bounds on the cockpit cross-section and cockpit volume.
For example, a TB20 does exactly the same MPG at 140kt IAS as a Cessna 400.
A Jetprop, if you can get the fuel duty free (no longer possible in the UK on private flights), and you fly it high up, etc, delivers a cost per mile which is similar to the piston singles.
Obviously in practice other costs will be significant. The scheduled maintenance on a TBM, even if never flown, is of the order of £40k a year - according to some owners I know. The capital cost will however dominate...
If you are after another data point, at FL100-120 I am burning 9.5 USG/hr for 140kt TAS. That is an "extra economical" cruise, 2200rpm, peak EGT. But with 4 people, on a long trip, they would get to know each other quite well 
My gut feeling is that anything with a Rotax 912 variant would do pretty well too. Particularly as they can run on mogas.
A Diamond Katana DA20-A1 does 117kt TAS at 7.500 feet, burning 4.3 USG/hr. Or, at least, that's what the specs claim. It's a two seater of course but in a lot of cases that's all you need.
At Wings over Holland a DV20 (the predecessor of the DA20) goes for 152 euros an hour wet.
But talking single engine... How fuel efficient would the latest ETOPS airliner be on one engine, loaded with pax? Even with one engine inop, my gut feeling is that they will still be more efficient than any piston pounder we can come up with.
By the way, the Robin Ecoflyer does 105 knots IAS on 20 l/hr of Jet-A or straight Diesel. At least, that's the data for the 1.7 Thielert/Centurion. The ones at my club all have 2.0 engines now but performance figures should be very similar.
And four seats of course, although with four adults on board you can't take a lot of fuel.
I saw some advertised data on the Boeing 787, and it was only 20% better than a TB20 in fuel burnt per passenger mile. Amazing really, and it shows how inefficient jet engines are compared to pistons.
One engine? Maybe a big jet pilot will join up sometime I am sure the extra drag will be quite significant though.
For slow therefore more flying hours per £1 ?
Almost any UK light a/c in Class II exemption from EASA reg's, with a reliable e.g. Rotax engine up front will probably yield 27 or so mpg.
Two seats, but often flown solo that's approx. the same mpg as my venerable Merc, C180 petrol car.
Third party insurance <£150 and MoGas, strip based, a Permit to Fly with some DIY and it doesn't break the bank, nor dominate one's outgoings.
The Jodel 1050/51 e.g.(Conti 0-220/Potez 4E20A)is a super 'plane cruise about 105 kt and can seat 2+2 or two only plus loads of camping gear. ~ 22 L/h.
This of course is the 'fun flying' end of the spectrum.
mike hallam
Slow designs have lower hourly fuel consumption and four-seaters with filled seats tend to do OK on seat-miles/gallon, notwithstanding the fact the in the real world wind kills the utility of slow aircraft and the back seats of four seaters aren't so often used.
If you decide arbitrarily that the ideais to carry two people a long distance, fast, and not use much fuel the Rutan Vari-Eze always seems to come out near the top of the heap: something like 180 mph cruise on 6 US gph, or in other units 155 kts on 22 liter/hr.
Some of the tiny SSDR aircraft should be very cost-effective. The Colomban Luciole apparently burns 6 litres per hour for 106 knots cruise and 4.5 lph at 93 knots. You can trailer it, so you don't need a hangar, though at the end of the day who wants to disassemble their aircraft every time they fly. You can maintain it entirely by yourself if you have the skills, so really your only costs will be fuel, a few spare parts, and the airport fees.
None of this is any good if you want to take passengers up in IMC, so at risk of sounding peevish, you need to be clear what sort of flying you intend doing.
Correction - units in my previous post should be mph, not knots.
