I think that 6 cylinder engines are perhaps the most musical of all engines (with the V10 next).
Why?
They sound good all throughout the rev range.
American V8's sound best at idle or low speeds.
European V8's sound best when at full throttle.
V12's don't sound great all the time either
A Porsche flat 6 sounds great at idle, great at low speed, great when climbing through the gears, great when at full tilt. It always sounds great. Other engines can't do the same.
Of course, I mention a Porsche flat 6 but other 6's seem to share the same qualities.
This is an opinion which cannot be proven so take it as such. You may disagree but that's my reasoning