Don't think i've ever bettered manufacturer quotes, but memorable:
Jeep Cherokee, 1998 S, new (for purposes of new, I mean had from new and within a couple of years old, not knackered) - 32mpg on a run delivering computers from Scotland to bits of the UK including doing the Woodhead Pass in snow, but primarily in 2WD. Also first time I sustained a trip-computer 4mpg for over a mile, flooring it up a hill.
VW Golf GTi Mk 2 - 8V 5 door, Digifant. (130,000 miles) Routine average 44.1mpg. Previous owner had got the trip computer to 27mpg average. I tried driving as fast as I possibly could up the A7 from Hawick to Edinburgh. Result was 47mpg (verified with fillup, but the computer was very accurate). When I sold it I decided to see what the best I could get on the computer was by being miserly, but not idiotic, and I got it to 52mpg.
Mitsubishi Carisma GDi (new) - usually gave 42mpg, not bad for a car that would do 90 down the Coldstream road 5-up. Orange light came on, reset trip computer - got 52mpg while nervously getting it to Jedburgh's station from Otterburn...
Citroen C3 1.6 HDi Airdream+ (new) - routine 65mpg. Mad-dashes dropped it to 50mpg. Attempted maximum economy/adhering to legal limits from Leics. to Newcastle - got 72mpg and very bored. I remember the manufacturer claim was 85mpg.
Did get my E320CDi (W210, straight six - OM616? I forget which is which, not the V6 and not the E300) - with 112,000 on the clock up to 50mpg but it was usually closer to 40.
Chrysler 300C Hemi - another trip computer in USGs, this car gave me 34mpg on the motorway in real gallons - was better at 85 than 70, multi-displacement system was good. Routine driving was 27mpg. And this was a car which I did not hang about in, but I am very chilled in towns, don't race from lights or anything.
Most shocking cars:
New Beetle Cabriolet - new - 30mpg average. That's what I get from my SLKs (I keep trying to push the 230K to 40mpg on long runs, but it can't quite get there and then it's quite thirsty in stop-start).
MX5 1.6 - 28mpg. That was shocking. Also utterly hopeless on the motorway above 80mph. Still would for country lanes though.
And I couldn't quite defeat the prodigious thirst of the 500 SEC, even with the driving style I have that lets me get good economy from others. All the driving techniques/habits in the world can't defeat a 5.0 V8, 4-speed old-tech slushbox and well over 2 tonnes of car. Reckon I'd have got a 5-speeder up to 25mpg though. I got the XJS up to 17.1 average and on one run, over 20 on the computer (soon eradicated by towns).