Been waiting for this topic. This can be difficult to judge with the three characters each having such fluctuating levels of power depending on choices throughout the game, classes, specs, etc. With that said, here's my examination of each of the three characters.
The Warden is treated like a wrecking ball nearly whenever mentioned. They have famed skill even before the games begin, often which has Duncan seeking them out in hopes of recruiting them. That's not much on its own, but then you see some of the abilities the Warden can obtain and some of the comments characters make. Being able to become an Arcane Warrior and a Spirit Healer and a Battle Mage and a Blood Mage. Or a Spirit Warrior/Reaver/Berserker/Templar. Or look at some of the Awakening skills, in which the character can undergo grueling meditation and training in order to reach untapped reserves of stamina and fortitude. Look at some of the fights. Sure, there's the basic assortment of pride demons, a Forbidden One, high dragon, all that fun stuff. But then it's not just any pride demon, but one so powerful they managed to pull an entire village across the veil. Not just any golem, but a 20 foot tall, superheated solid iron golem. Not just any high dragon, but one touched by the fade. Oh yeah, and a couple gods and an ancient magister as well. We get characters like Wynne telling us she'd never seen anything nearly so terrifying and ferocious as the archdemon...except the Warden. We get Cassandra looking for the Warden first above all else. We get others calling this character a one man army. We might call into question the Warden's ability to lead an army in comparison to others, but getting down to it I think it's difficult to argue that this is THE badass of the Dragon Age universe.
Hawke... Thing with Hawke is it has always been difficult to say where Varric's exaggerations end, exactly. Did he kill 50 people every time he went down the street to pound back a couple pints? Doubtful. Did he duel the Arishok? Yes. I don't think much a case can be made for Hawke as the strongest character. Aside from their story being absolutely littered with personal failures, I think it's rather telling that the stories that Varric recalls first (should the worst happen) is not some grand martial feat, but rather confounding an opponent without ever drawing steel. Is Hawke a strong, skilled character? Yes. Is Hawke one of the more powerful in the land? Likely. Moreso than the other two in question? I very much doubt it.
The Inquisitor is a more fun one to think about. This is a character who, especially with the right choices made, could have godlike power. Thing is, the character isn't really interested in that sort of power so it's likely to always remain untapped, if still deadly. But let's back up. Who is the Inquisitor? Nobody really knows, or cares, until this game. They were a forgotten son or a castaway, perhaps even disposable...until they gained the mark. Now through training and education this character can be built up to something much greater, but it's never to the level of "The character has learned to find reserves of vigor in personal suffering, granting a further bonus to mana or stamina." Meanwhile, the character surrounds themselves with a rather impressive cast and depends upon them quite a bit to get the job done. So in terms of pure martial ability I actually find it likely the Inquisitor is the weakest of the bunch. Not unskilled, mind you, especially not after training up, but also not as naturally talented or focused as Hawke or the Warden. It's the mark which throws this discussion for a loop. How powerful does the Inquisitor allow this ability to become? How effective would it be against the others? And would it be enough to overcome them? Certain enemies seem to have more resistance to it, certainly, it's not some instakill ability the Inquisitor is capable of utilizing. It seems tied in most to willpower, with the more an opponent has, the greater they are able to resist its influence. As such I think the mark would be enough to easily put the Inquisitor over the top of Hawke, who though perhaps able to resist, would be so weakened so as unable to compete. The Warden? With things like this under their belt: "The character has begun a regimen of strenuous exercise, the character has suffered intentional injuries in order to develop an innate robustness, the character has embraced ancient techniques of meditation, the character has braved exposure to the elements in order to build mental focus, the character has learned to find reserves of vigor in personal suffering..." I think the mark would have a difficult time effecting them fast enough for the character not to stomp them into pudding one-on-one.