According to wiki spirits of Hope are strongest, at least among those revealed. Also according to WoT "Spirits of Hope rarely appear in the waking world since little there attracts them". I find this interesting to be honest. Nothing attracts them because there is no hope in the world?
Because Spirits of faith are also known to be among the strongest but there is enough faith in Thedas to attract them.
I'm also wondering about how Tevinter would treat a mage who is possessed by a spirit. Because practically that mage is immune to tranquility and has unlimited connection to the fade, that is like you have infinite mana in the game. We know that Lyrium is used in rite of tranquility, if a possessed mage is immune to that, this should make him automatically immune to Templar abilities as well, since they use the Lyrium in similar way to interrupt magic. The only real thing we have on this is Fenris telling Anders he should go to Tevinter because they would appreciate him over there, pointing at the fact that he is an abomination who is in control, at least mostly.
This is one of the thousand questions I want answered when Tevinter comes. 