For me, it's the Porsche 911 Carrera RSR Turbo 2.1 "R5"
>first turbocharged 911 in the wurld
>built out of a regular RSR in anticipation of Group 5 regulations
>entered as a prototype
>small displacement, big turbo, 500hp
>917 wheels and suspension components, around 800kg race ready
>P2 in Le Mans 1974