Horizontally opposed engines offer a good low center of gravity, make distinctive noise, and when six cylinders are used offer great smoothness, they're also relatively compact in length.
But my favorite engines are 2-strokes. Without any valves, or cams, or timing gears, they're mechanically simple, and mechincally quiet. They have very smooth torque and good power output for their size, and they're light compared to 4-stroke engines.