And the winning combo is…. 2em and 3 explosive. These types of drones are faster and have better tracking. The em drops the shield faster and the explosive chews up the armor. There are specific ships that are not a good match for this combo such as the pilgrim. However, as a standard the em ex damage drones outperform other combos over most of the common racial ship types.
The eve uni site has some great table showing the speed and tracking advantages of em and explosive damage drones. Well worth the read.