Tevinter - 1st if we are talking military power and darker magic (I.E. more unique magic)
Circle Mages - 2nd in military power and conventional magic (I.E. not much variation outside the normal spells)
Sarebass - 3rd in military might (alone, 1st if accompanied by a group of qunari warriors), non-conventional magic, but limited magic
Dalish - Not enough mages to have military might, unique magics
Witches of the Wilds - no military might, as they do not participate directly... old, powerful, unique magics
Mortalatasi - Very little military might, unless with an army of corpses, unique necromantic spells.
Rivanni Seers - no military might, unique spells usually not suited for combat
Darkspawn Mages - tied for 1st in military might, taint induced magics, can spread taint...
its hard to decide who is more powerful if we dont give them a set category to compete in, as all groups will have their strong points in their respective fields