Fair Fight specifically copies the attack stat of the unit that is attack or retaliating against it. Attack upgrades are counted, since they modify the unit's attack stat. Talents that modify the damage dealt do not alter the landmark minion's attack stat. 

For example, if a landmark minion with Fair Fight is attacked by a unit with an attack stat of 2 and the talent Hunter (which allows it to deal an extra damage when attacking a landmark), the landmark minion will take 3 damage but its retaliation will still only deal 2 damage.