My finger was really gunning for the BMW inline 6, but I settled on the Porsche boxer 6 in the end.
It's just so all-round awesome. If I got this right, its been around for years in anything from a 956 to the 911GT1, Dakar, the 959... That's track, off-road and road.
Its been air-cooled, liquid-cooled, carburetted, fuel-injected, naturally aspirated, turbo-charged, super-charged, duplexed to create the 917's flat 12, been rear-engined, mid-engined and probably front-mid engined as well, and a whole list of other dictionary words I'm not familiar with.
It sounds great, goes great, looks great and if what I hear is true, is bullet-proof.