We like engines with complicated histories (such as the Buick/Rover 215 V8), and the Peugeot-Renault-Volvo (aka Douvrin) V6 certainly qualifies.
The PRV V6 was manufactured for a quarter-century and powered some pretty cool machinery: the Volvo 262C Bertone, the De Lorean DMC-12, the Renault Alpine A310, the Peugeot 504, and the Citroën XM, among many others. Its development was full of dramatic plot twists; originally planned as a 90-degree SOHC V8, the 1973 Oil Crisis threw a big metric monkey wrench into that plan and led to the engine being hurriedly redesigned as an odd-fire V6 (sound familiar, Buick V6 fans?). Even-fire, 24-valve, and turbocharged versions followed. The PRV never established a great reputation for reliability, but it got the job done for a whole generation of European vehicles, inlcuding some 24 Hours Of Le Mans cars in the late 1980s.
[Douvrin PRV V6 Resource Center, Wikipedia]