I don't use mrecenaries too much, but when I hire them, it depends on the faction I'm playing.
Playing as the Byzantines, for example, I usually hire feudal sargents, chivalric sargents and pikemen, because the I can only train simple spearmen.
As a Catholic faction, different horse archers units can be my choice of mercenaries.
But on the whole, I mostly try to do the job with the national troops.