The intake howl from the four Bristol (!) Olympuses in the Vulcan always gets to me.
The Merlin as already mentioned a few times. I was standing outside a warehouse near Hucknall in about 1990 as Rolls-Royce's Mk.XIV Spitfire flew over. 'Nothing like the sound of a Merlin' said the bloke standing next to me.
I couldn't say I could identify a Merlin from a Griffon heard singly, but I do prefer the Griffon growl and seriously lumpy idle.