Best unit is beyond discussion the "redcoat"
Its just terrible unbalanced.
its the unique unit for englan empires and they get it instead the rifleman.
redcoats have 2 more power, wich is cool but they also have 25% against mounted units (as the riffleman) and 25% against gunpowder units.
Basically they have 25% more against everything they should fight against.
but what really makes redcoat great is that they are NOT riffleman so grenadiers dont get the 50% bonus for attacking them, there isnt anything of the same technology level that have any chance against that unit, even if upgraded (for example, a riffleman needs combat III for having something near of redcoat power and combat IV for having more than a 0 experience redcoat).
anyway i think my favorite unit is chu-ko-nu. chinesse special unit, its a very versatile unit having 1 or 2 attacks of initiative and colateral damage (wich makes them being usefull even against adavanced technology as kamikaze units) but not as unbalanced as the redcoat.