Well, Inquisitor defeated to defeat Corypheus, the one of Seven first (and propably the most powerful) darkspawn using his/her Anchor, but the Warden was able to kill Architect, the other one of Seven without such powers. Corypheus wields an Orb of Destruction, which according to Solas gives him power of elven gods, so he could use this power against the Inquisitor and lost, Warden defeated Flemeth, who also was an elven god (oops, spoilers).
Warden is far more skilled using many kinds of weaponary than Inquisitor. Warrior-class Warden can use both-handed weapons, weapon and shield, dual wielding weapons, daggers and even bows or crossbows, while warrior-class Inquisitor can use only both-handed wielding weapons or shield and weapon. Same about rougues. Plus, Warden can specialize in two of four (or even three of six, as Warden-commander) specializations, while Inquisitor can choose only one.
And I think the Archdemon is more powerful than the Red Lyrium Dragon and any other dragon killed by the Inquisitor. According to Wiki, Archdemons are far more intelligent than average dragons.
Hawke is here outmatched, one of his/her greatest feats is killing the Harvester, but Warden also killed such creature. Other of great feats of Champion is defeating Knight-Commander Meridith wielding swod with Red Lyrium idol. Idol was only a little piece of Red Lyrium. Inquisitor defeated numerous red templars and behemots, MADE of red lyrium and even a Red Lyrium dragon.
In the beginning of DA2, Hawke and his/her sibling was outmatched by darkspawn, and Warden and his/her three companions cut through the Ortan thaig, Caridin's cross and entire Dead trenches on their own...
At the end, I think the Warden is most powerful.