For me, it's the Aston Martin Valkyrie, by a long way. Let's start with the looks, it is absolutely stunning from every angle, the rear angle is particularly incredible, the lines and shaping are just unrivaled in the class, chuck on the classic AMR British racing green livery and it's just perfection.
Then there's the concept, they looked at what other manufacturers were doing with their engine concepts with the various twin turbo V6/V8s, single turbo with big electric motors and whatnot and decided that none of that was really for them, and went with a naturally aspirated V12 with no hybrid system, just straight up, raw, unfiltered NA power. That also helps to deliver the pièce de résistance, the sound, nothing else in the class sounds like this does, and potentially nothing in current global motorsport does either. It sounds like all the best F1 engine notes rolled into a modern package, the shifts sound incredible and the fly-byes are what motorsport dreams are made of. If you ask someone what the ideal racing car engine/exhaust note is, there's a good chance it'll be very close to the throaty roar that the Valkyrie produces.
Honorable mentions, the Ferrari 499P, really is a beautiful looking car still, has great lines,and the liveries always work so well on the car. Peugeot 9X8, dared to be different, radical looks, the lights really stood out and the liveries are always wild. Finally, the Cadillac V-Series.R, similar reasoning to the Aston in that they decided that what was really needed was a naturally aspirated monster of an engine, with a teeny tiny hybrid motor to get it moving, that noise from electric to combustion is one of the greatest single momentary sounds in the history of motorsport.
