It has to be the Bugatti Royale.
I saw the replica they have at the Donnington Motor Museum and it's totally bonkers.
A few facts:
- the front bonnet (hood) is 7ft long and the engine is 5ft long
- the engine is a 14.7 litre 8 cylinder, the crankshaft alone weighs over 300lbs
- the car is 21 feet long and the wheel diameter is 38 inches!!
What an exceptional car