The Inquisitor is, without a doubt, the most powerful. Honestly, aside from your phenomenally good (or bad) luck, you had a mountain fall on you and managed to walk hundreds of feet through a snowy mountain and live: Inquisitor has some crazy good endurance. The Inquisitor is shown to be much more cunning and quick than our previous Protags. The Inquisitor asks FAR more questions than any other Protag and can get a grasp on a situation long before others can. Quizzy also thinks quickly: Opening a rift to keep from falling to their doom, kicking a harlequin out a window, opening another rift just to screw with Florianne's people, opening a rift basically inside of someone to suck them into the fade, etc. And depending on how you play the game, you can kill: 1 Envy demon, Twice travel through time, Kill an army of demons, kill the Nightmare, defeat an ancient blighted magister and his blighted red lyrium dragon, and then go and kill ELEVEN high dragons. Eleven. One of which is possessed by a spirit so is technically an abomination High Dragon. Then you can go down and kill HUNDREDS of dark spawn in the very deepest deep roads, defeat a Titan, the list goes on.
I'm also going to argue that the Inquisitor's companions are the most powerful out of all the companions. Anders/Merrill/Old lady Wynne don't hold a candle to Vivienne/Dorian, two mages of exceptional talent who have been raised and taught from a young age. Varric is..Varric. Sebastian is a chantry boy, he doesn't have nearly as much training or experience as, say, Blackwall. And Alistair was barely a new Warden, sure he had some templar training but any of the warriors could own him. Avelinne is tough, but is she Cassandra tough? Iron bull and Oghren would just go out for drinks. Cole can literally vanish, Inquisition-Lelianne is 10x more badass than Origins Lelianne. Zevran's good, but I doubt he'd be willing to fight this theoretical fight. He'd go hang out with Lelianne. The only two people who would be of some concern would be Fenris and Shale.
For everyone who wants to say Morrigan would be the end-all-be-all for the Warden's victory, one word: Solas. He doesn't even like her, especially after the events in the Arbor Wilds.
And I want to say, depending on things, the Inquisitor has a lot more loyalty going for them than the other Protags. I mean, Morrigan will leave you at the drop of a hat, Zevran can abandon you, everyone in DA2 can hate Hawke and possibly leave, etc.
Thats not even getting into the whole, nation-sized army.