Answering in context of cars I have experienced where the sounds, performance etc of the engine are a major part of the character of the car (although I can't help think of my bevel ducati with contis when it comes to character). Already mentioned, but huge capacity aero engines with some wheels and a seat and not much else added and from Edwardian times - its all about the engine! Best collection I have seen is Prescott hillclimb in UK. Visually I remember this Bentley with 12 cyl Napier engine, set out like a v8 but with another bank of 4cyl sitting in the valley. See photo from lhs with two exhaust and another on the other side. But also I did some marshalling in trials for the VSCC and I remember a 20 something litre aero engine put in a 1911 Mercedes I think it was, but on this wet hillside in the woods, I could feel the vibrations through the ground before I heard it and realised it was a car. Personal favourites, rotary engine in my RX5, lotus twin cam in my mates Escort, Alfa 1750GTV and 4AGE toyota engine, first in 87 in my Corolla which was pretty exciting, real sleeper, and more recent development with 20v version in my Asp, where the combustion chamber is just a circle of valve holes. Less obvious, M130 Mercedes - just the normal 6cyl OHV that powered so many cars from 60's and striking similarities to Datsun 1600 etc engine, but in a 68 280sl, with manual trans, it feels fairly normal puttering around but up in the revs it starts growling and pulling much harder and is a big part of the character of the car - not just the pretty body most people see and I am sure most never explore the upper rev ranges. Another that probably doesn't stand out as one of the 'greatest' but significant part of the character of the car, 5.6l V8 in my W126 560SEL. Lovely silky smooth puttering around and always feels strong, like you would expect a big luxury car, but plant your foot and it takes off with quite different character and pulling right to quite a high redline. I remember a Top Gear where they listed 9 out of 10 of the worlds dictators had one - I guess you need to take off quick if people firing bullets at you! And reading about peripheral ports on rotary engines and how mods can change engine character. The B series 1800 in my Bacchus feels quite different to any other B series I have driven, the engine modified by Bill Hanson in Adelaide who was an MG specialist, that bought the chassis in 1963.But also I have never driven a B series before that is only pulling around 520kg of car! Bigger scale modifier, 'premerger' AMG took cars from the factory and considerably changed their character (at a price!). 5.0l V8 in 1982 500SEC and they stripped and blue printed the engine, bigger inlet valves, porting, different cams with earlier model solid lifters and lightening of valve gear. Also minor details I have discovered like handwriting on side of the fuel pump relay that they modified to allow a higher redline before fuel cutout. Under the bonnet you still couldn't tell from looking and the hidden mods are not exactly rocket science and extra 50hp, but it considerably changes the character of the engine and car. I can't find an image to post, but the dyno graph in advertising is just how it feels, similar to standard in lower rev ranges but where the curve starts to flatten, the modified engine is a much straighter line all the way to the redline. It feels a bit like my old rotary where the power seems to build and build without tapering off.