Ferrari 250 and 330-series V12 engines are super-sweet sounding. E46 M3's, Tadek Marek-designed Aston straight sixes, and the Maserati 3500 are also great. I also really enjoy the pounding of an American big block six. Aside from that, the old 200ci Ford flathead fours (yes, the ones in model T's and B's) can be made to have a neat-o sound, and of course I love a Ford Flathead V8. Lastly, I think that most VW 4-cylinders are not bad sounding at all.
Oh, and I almost forgot. There's a video on YouTube of a guy running a supercharged Packard inline 8 in his hot rod. Great sounding car.