Durzo or Kylar would be the strongest, Durzo explains that all the Ka'Karifers (sorry if thats spelled wrong) have massive talent but no ability to use it without the bridge in their talent that the Ka'Kari creates, second would be Dorian it seems, though his talent (i keep trying to call it chakra lol) is obviously not even as strong as it used to be as we're told that the Vir reduces the users power with talent, after Dorian i'd be hard pressed to say who it was due to there not really being good ways to compare, feir doesn't seem to come into it but Solon and Vi both have some serious power and this comparison is made even more difficult when you factor in all of the Vir users too.
Oh and don't forget what sister ariel said about Ully she has possibly more untaped power than Vi.
depends on what exactly you're asking for strength. if you're being literal and asking about pure magical muscle then i would say Durzo, Kylar, or Vi. if you take into acount the black Ka'kari then most definately Kylar since he can absorb magic using it to stregnthen himself temporarily but if you're talking overall strength then that will take into account knowledge of how to put it to the best use which i would say would be Durzo due to his slightly over the mountain age of 600ish (haven't read the books in awhile) after all they say in the books that the strongest mage magical muscle wise was jorsin but he had to balance magic training with kingly training while the second most powerful mage Ezra was completely devoted to magical training which puts him ontop of Jorsin.
Both Kylar and Khali are all described as Archmagi.. The Speaker described Kylar's power as a teenage Archmage unaware of his own power.. and Kylar saw Khali as an Archmagi when she possessed Elene.
Feir thought Vi was as powerful as Solon but was highly intimidated after seeing her talent... Durzo never hit his peak so:
3. Durzo (+ Dorian with Vir)
7. Dorian (Without Vir)