Early in my campaign I got a lot of Mercenary Crossbowmen and they were really good to have until I got Scots Guard (very good archers!) and Aventurier (the best crossbowmen in the game!).

But my favorite mercenary unit so far is Alan Light Cavalry! As French I have access to a lot of heavy cavalry (Chivalric Knights, Lancers, Knights Hospitaller and Knights Templar) but I have no fast light cavalry at all, so the Alan Light Cavalry is very good to have! Just as sharpshooter said they're perfect for catching annoying horse archers and fleeing cavalry that your slower heavy cavalry cant outrun.