What is best unit to hit plots in rein of chaos?

To take an unoccupied plot you can use any units as long as the legion is strong enough (depends on the level of the plot). Higher tier troops, more troops and better heros skills will all help.

If you are trying to take back a plot that is already occupied then you will need to do 500 points of demolition damage to it. You can check your legions for their demolition value to see how many marches it will take. In this case the best units are the footmen. They have the best demolition ratings. A large march with all Footmen of a high tier can do over 500 damage to take the plot back in one hit.