With regard to the new poll: I admit to favouring the 'tone' of the PR 100.2s. Long before I ever thought of being a driver (say late '80s, early '90s), I'd hear them as newer buses rolling by in the distance. Their 'note' being distinctly different from the Mercs and MANs of the then ACTION fleet. Just every now and then you'd hear one let off its familiar 'air fart' as it zoomed along. Still a beautiful thing that.
I'm also a fan of the MAN diesels. An even tone and quiet too. As you'd expect Iris's get a big thumbs down from me. To my ear they sound like a broken down washing machine much like the one we have at home here. The CNGs also have, IMHO, an annoying idling tone. It's the way they kind of 'cycle' as they idle. Listen and you'll get the idea.
Agree, the sounds of the standard PR2s are hard to beat; used to enjoy the whistling sound that the Scania CNGs made (especially when decelerating) in their younger years.
PR 180.2 Artics > PR100.2 > MAN Diesels for me. Aside from the .3's, the rest aren't that nice.