The Triwizard Tournament is an infamous competition between wizarding schools in the Harry Potter universe. The tournament's first (and only) enactment is in Harry Potter and the Goblet of Fire, the fourth of the series. One of its explored questions involved evaluating who earned their place as a Triwizard Champion. While Harry is often the remembered victor of the 1994 Triwizard Tournament, the other Hogwarts Champion, Cedric Diggory, is the one who should be solely credited with winning the tournament.
Prior to Harry's surprise entrance, Cedric Diggory was formally named the Hogwarts Champion. Cedric had been chosen alongside Fleur Delacour from the Beauxbatons Academy of Magic and Viktor Krum from the Durmstrang Institute. With Harry's questionable addition, many began to resent him for doing what so many were prohibited from being able to do. Naturally, a majority of the school began to support Cedric, who they believe is the rightful Hogwarts Champion.
Despite Harry's questionable entry and Dumbledore's decision to allow him to compete, Cedric's name was still chosen to be the Hogwarts Champion. When Cedric's name was spit from the goblet, it did so with the knowledge that Harry's name was inside. If Cedric didn't have a legitimate standing or chance to be the Triwizard Champion, then the goblet wouldn't have pulled his name to begin with.
When nearly the entirety of Hogwarts rocks "Support Cedric Diggory, The Real Hogwarts Champion" buttons that flashed to a "Potter Stinks" reminder, Cedric never vouches for the buttons. In fact, Cedric never openly mocks or distrusts Harry throughout the tournament. While Cedric may not entirely believe that Harry did not put his own name in the goblet somehow, it's important to note that neither did Harry's best friend, Ron Weasley. Despite Harry's controversial presence in the tournament, Cedric never treated Harry as if he didn't belong.
As a Hufflepuff, Cedric is already expected to uphold and embody a strong sense of loyalty and morality. Over the course of the Triwizard Tournament, Cedric does just that. Cedric never acts as a vindictive competitor nor is he overly boastful about the abilities he is clearly well skilled at. He remains level-headed and attempts to win the tournament fairly.
In the first challenge, Cedric was able to ward off his dragon, the Swedish Short-Snout. While Harry did warn Cedric that the first task required each champion to defeat their own dragon, Harry had an advantage. Severely unprepared for the tournament, Harry was guided through the tournament by Barty Crouch Jr. (disguised as Mad-Eye Moody). With the other champions guided by their own headmasters and with Dumbledore (and many other professors) seemingly supporting Harry, Cedric had to rely on what he knew and the few higher-ups that wished to help him out.
Prior to the second challenge, Cedric cracks the dragon egg and its secret (with the help of the distinguished Barty Crouch Jr.). Arguably, without the advice from the disgusted Death Eater, Cedric wouldn't have figured the egg out in time. However, Cedric earns moral points by taking what he has learned to Harry. Cedric reasons that this is a repayment for Harry warning him about the first task. He credits that warning with giving him time to prepare in order to defeat the dragon.
Yet, it is proved throughout the tournament (and through others' views on him) that Cedric is an extremely talented wizard. So, it is possible that Cedric may have been able to accomplish the task by thinking on his feet and by using the transfiguration spell that helped him get the egg in the first place. Despite receiving assistance from Barty Crouch Jr., Cedric does use his own wand to cast a useful bubble-head charm during the second task. Harry, who is also assisted by the Death Eater, struggles to come up with a solution that would allow him to enter the lake.
Inside the lake is where Cedric and Harry's paths cross. Cedric, who has already freed Cho, stops to warn Harry that their time to complete the task is almost up. With Fleur out of the running, it is only the two of them competing against Viktor. So, while Harry and Cedric are competing against each other, Cedric remains morally righteous and offers help to Harry without feeling as if he's repaying him for something.
Prior to entering the maze, the final challenge, Dumbledore warns the four champions to be cautious. He insists that "people change in the maze" and that if they aren't careful, they may lose themselves on the path to finding the cup. However, Cedric's morality remains intact. This makes him a strong candidate for the cup because he has earned first place without direct guidance from someone else.
During the final challenge, both Cedric and Harry spot the Triwizard Cup. The other two contestants, Fleur and Viktor, didn't stand a chance. Fleur was already behind by entering the maze last, but she is sabotaged by Barty Crouch Jr.'s interference. Viktor, who was bewitched with the Imperius Curse, was a pawn in Barty Crouch Jr.'s game to keep not only Viktor out of the game, but to try and stop Cedric as well. Attacked by Krum, Cedric faced a number of challenges within the maze. Harry on the other hand, didn't face any true forms of danger unless he attempted to aid his competitors. Of course, the only reason they are in an exceeded amount of danger is because of Harry's addition to the tournament.
In the end, Harry and Cedric debate over who should take the cup. Cedric suggests that Harry should take it because of the help Harry has given him throughout the tournament (as well as the hep he's given the other competitors despite how he's been treated). However, Harry also recognizes what Cedric has accomplished in this tournament and that he is an incredibly skilled wizard that deserves to win the eternal glory that comes with winning the tournament.
Cedric's suggestion that Harry should take the cup can be chalked up to Cedric's modesty and kindness (both of which are Hufflepuff traits). Cedric has not acknowledged his own accomplishments throughout the tournament and believes that Harry saving him from Viktor Krum has earned him the Triwizard Cup.
Ultimately, Harry and Cedric decide to take the cup together, but it is Cedric who recognizes that the Triwizard Cup is actually a Portkey. While he does not initially recognize the danger he and Harry are in, Cedric does attempt to defend Harry. In fact, Cedric loses his life while attempting to protect Harry from the dangerous situation the Portkey has transported them to.
Cedric's story doesn't end at his death, as he helps Harry escape Voldemort's clutches (and therefore, death) in the graveyard (along with a few other notable people from Harry's life, including his parents). Despite his death, Cedric and Harry are crowned co-Triwizard Champions. Considering that the Triwizard Tournament was quite literally rigged to guarantee Harry's win, the fact that Cedric still managed to come out on top attests to how strong of a wizard he was.
Cedric may lack the flashiness of his competitors but his hard work, loyalty, patience, and modesty, were traits that made him worthy enough to become a Triwizard Champion. He quite literally defies all odds by making it to the end of the final challenge of a tournament that has become considerably more lethal. The tournament may not be a test of morality, but it is a test of how strong of a wizard one is, which can take a variety of forms (as determined by the sorting of Hogwarts Houses). Yet, Cedric is simply the rightful choice of champion because he played a fair game that ultimately cost him everything.