Here is my vote for best sound R/C aircraft propulsion.
1.Kolm 200cc inline 3 cylinder
2.Moki 400cc radial
http://www.youtube.com/watch?v=oHfvbxim2_o
3. any of the high blade count EDF in High Bypass scale plane
http://www.youtube.com/watch?v=TK5Hd3RnNJk
3. turbine
4. saito 200 Vtwin
http://www.youtube.com/watch?v=wBickqgN_fM