govenors with a MINIMUM acumen of four are best (more if you can find one).

once i get a new govenor for a province i immediately move him to the same stack as my king, i tend to get fewer vices that way and it helps loyalty.

i never put them into combat, i just use all of the men in the govenors unit to replenish depleted units, that way i usually end up with a couple of stacks of 1 man units, all of them govenors and have them follow the king around wherever he goes.

as for the loyalty, piety and dread, well i try to keep them at a miimum of 6, 5 and 3 where possible, but acumen is the determining factor for me.