Hm, let me think a bit..1) Judging by the amount of killed dragons:Warden - 3, Archdemon includingHawke - 1, just 1Inquisitor - 10 + 1 corrupted by CorypheusWell, Inq seems to be greatly powerful at killing dragons. At least in direct competition with Hawke, Warden may just like "quality over quanity" way of things.2) Jusdging by deedsWarden - ended fifth blight, one of five people that have ever accomplished so. Also did it faster than any of them.Hawke - formally started an international ciwil war, bring the plague of Red Lyrium to the world and beat Arishok in 1 on 1 combatInquisitor - killed an Ancient Magister who once started all Blights and threatened to bring never ending chaos to Thedas. Also bring an end to an international ciwil war.Once again, Inq won the competition.3) Now let's take a look at the pretenders themselves. Their social ranks are quite high:Warden - Warden Commander, very powerfull figure amongst Grey Warden order though not the leader of it.Hawke - Champion of Kirkwall, the protector and warden of one city. Not most rich city neither most powerfull.Inquisitor - Leader of the Inquisiton which military and political strength could put to shame the entire empires. Status of the Holy and Saint organization gives some extremly unique possibilitys. You just can't deny them whoever you be.Inq won! Surprising, isn't it?4) And finally, let's see who they are without all the social statuses:Warden - a Grey Warden who survived ritual. He can sense darkspawn and good at killing them. Also can kill Archdemon, like any other Grey Warden. Well, thats it - he one of tousands wardens.Hawke - Just a random dude, nothing really special about him. Skillfull warrior and thats it.Inquisitor - have an ability to enter the Fade physically anywhere anytime. Can also use Orb of Destruction's magic to manipulate the Fade for his own benefit.Inq. Yeah, he won again.Don't feel that bad, he was just designed to be "The Most Powerful Dude in whole Thedas". I'm sure next DA will bring us even more badass character, so...
...where to starts...
1) Maybe because there are more dragons in DAI? Put 100 dragons in DAO and DA2 and Warden and Hawke will kill those 100 compare to Quizzy's 10. Beside, 1000 Cuts, -cough-
2) Using the Mark to killed Cory is kinda cheap considering that the Mark doesn't contribute to how skillful Quizzy is. I mean Hawke killed Cory in Legacy, give him/her the Mark and Hawke can do the same. Beside, Cory is nothing more but a puppet for whatever cooking for future games. Warden also ended a Civil War. Hawke stopped a Qunari Invasion.
3) I guess social status counts...then again Warden and Hawke actually worked for their title of HoF and Champion, whereas the Inquisitor is named Herald because of your glowy hand. Also, Cass said that their first candidate for the role of the Inquisitor is the Warden, followed by Hawke.
4) Again, by using the Mark. Without the Mark, the Inquisitor is just an average guy/gal. The Wardens is skilled, which is why s/he was recruited. Hawke, well, I guess surviving 7 years in Kirkwall counts as something.
So basically, almost everything Inquisitor has over Warden and Hawke, according to you, can be answered by the Mark.