The late 黄家驹 (Wong Ka Kui / Huang Jia Ju). He puts his heart and soul into every song voice evokes emotion. This is his trademark song, but he has so many other great ones.
http://www.youtube.com/watch?v=iA_qIycirRA
I also like Eason.
Jacky and Hacken are great singers, but to me their voices don't have the emotion that Eason's has.