Weirdly enough, I thought it was going to be a tough decision. Do I choose my LIs? Do I choose the best party? What about balance?
But then it all resolved itself so easily that it's almost weird.
My canon characters are almost always damage dealing warriors (dual wield in Origins, 2h in DA2 and DAI). So I need a sword and board, need a rogue, and need a mage. In step the strongest three, all conveniently my LIs from each game:
Warrior: Cassandra
Survivability, grabbing aggro, just enough Templar love to deal with magic users and have an added dimension to her fighting
Rogue: Isabela
By DA2 endgame, dual wielding Isabela was a walking harvester of souls. Add to that the best party banter, and this is a no brainer.
Mage: Morrigan
Tactically... who even needs heal? Storm of the Century, Storm of the Century, Storm of the Century, then step gingerly around the corpses. Throw in some Crushing Prisons and the occasional Cone of Cold combo break with a shield bash, mighty blow or back stab, it's just too much fun. And her voice is just the icing on the cake.