(Non-Pilot disclaimer)
Of the aircraft on which I've flown, the most impressive engine sound was on a KLM 767. I can't be sure whether it was a 767-300 but I'd imagine it was - was in about 2009. Thing is, I believe they can have either GE or Pratt engines. Not sure which one it was. But, yeah... KLM 767
Really incredibly loud and produced that 'saw-like' sound at high thrust settings that I love so much.