In theory or in actual gameplay?
I mean, in theory, Morrigan has incredible knowledge and skill: to know how to use the Eluvians, to perform the Dark Ritual, so on. Anders is capable of amazing feats of healing and has that nice/crazy spirit/demon inhabiting him that allows him to pull off some strange tricks as well, including temporarily reversing tranquility before it was thought possible. (Also, in origins/awakening, either can perform some very nice OP combinations.) And, as said, Solas is, well, Solas.
Having said that, even though magic was so reduced from Origins/Awakening to DAII and then to DAI, in gameplay, Viv could probably take them both in a cage match, simply because knight enchanter is the most broken thing around: much more then shapeshifter and spirit healer. Even if either of them picked up arcane warrior as well (which was pretty darn OP as well), knight-enchanter is just broken.
...Having said that, I'd rather think Morrigan or Anders (or Solas, though he never really shows super power) is more powerful, so. You know.
And then you have DAO archers who could basically destroy everything in sight... does that mean Leianna or Nate is the most powerful? Or pre-patched Sera's fire flask/thousand cuts abuse? Or are we going on story, in which it's probably Cole: who can make people forget him and basically go invisible at will (that would be so useful during battles...).
Again, warriors: Cass claims she can boil the lyrium in someone's blood, right? Wow. That's pretty OP and useful. But it never happens in gameplay, and Blackwall's Champion specialization is generally more useful for tanking then her Templar one, even just in DAI, and if you're after damage, Iron Bull's Reaver is better. Fenris being able to rip out hearts is pretty awesome as well, but that only happens in one cutscene (maybe two, depending on your choices?). Honestly, though, I'm unsure which warrior would dominate if you cage matched them, though. In general, DAO characters did less damage then DAII and DAI characters, but they also had more skills and strategies...