Slayer expreience goes by the monsters' number of hitpoints. Therefore you would want o search for the mage witht he highest number of hitpoints ,which in GodWars Dungen is Bandos's.
Here is a list of the HP and levl of all Spiritaul Mages:
122-Spiritual Mage -Armadyl-86
121-Spiritual Mage -Bandos-106
121-Spiritual Mage -Zamorak-75
120-Spiritual Mage -Saradomin-85