Prince Zuko starts off as the primary antagonist of Avatar: The Last Airbender, what with his fervent need to apprehend Aang so that Father Lord Ozai would welcome him back to the Fire Nation with open arms (not to mention returning him the right of succession.) In his quest, he undergoes considerable amounts of personal struggle, often wavering towards the side of evil but eventually realizing the necessity of balance in the world.
Zuko becomes the Fire Lord after Ozai is permanently subdued, and rules for several decades before graciously vacating the throne so that the younger generation could play a role in governance. This doesn't mean that he stops fighting, though — once a Firebender, always a Firebender.
10 Vs. Zhao (Northern Water Tribe)
After Zhao "kills" Tui, the Spirit of the Moon, the night is plunged into an eternal darkness. Following this, Princess Yue takes its place in the sky, while Aang and the Ocean Spirit meld together to fight off the invaders. The Admiral senselessly runs around, freaking out at the world collapsing around him, when he meets Zuko on an ice-bridge.
They indulge in a short but strenuous battle, with the young prince quickly gaining the upper hand (as he did during their earlier Agni Kai.) However, the event is cut short when La grabs Zhao and pulls him into her depths forever.
9 Vs. Azula (With Uncle Iroh)
Ozai sends Azula on a mission to retrieve his brother and son, telling her to use any means necessary. She, obviously, opts for deceit, claiming that the Fire Lord is ready to forgive all past wrongs and accept them both back with warmth and kindness.
The ruse fails when one of her crew members accidentally refers to them as prisoners, forcing Zuko into fighting his sister. She proves herself to be far more skilled and agile than he is, requiring Iroh's last-second intervention to save him from being speared by a fork of fiery lightning.
8 Vs. Earth Kingdom Mercenaries (Zuko Alone)
On his travels through the hinterland of the Earth Kingdom, Zuko finds that the Fire Nation is universally despised wherever he goes, often more than the Earth Army renegades who roam the land harassing and extorting poor people.
It is one of these bands of roving mercenaries that Zuko develops an animosity for, which culminates in them kidnapping his new friend, the little Earth Kingdom boy, Lee. Unfortunately, Zuko fails to win against Gow with just his swordsmanship, but when he resorts to Firebending, he emerges triumphant but is immediately ostracized by those who had just welcomed him the previous day.
7 Vs. Azula (With Team Avatar & Uncle Iroh)
After Azula torments Team Avatar with her incessant stalking, they try to divert her attention in such a way that she would end up facing Aang. The Avatar is exhausted after having very little sleep, but luckily he is soon joined by Zuko, who intends to snatch him away from his sister's devious jaws and claim all the glory.
The two boys are evenly matched against Azula; to be honest, it seems that she is about to win, but the tide immediately turns in their direction when Toph, Katara, Sokka and Iroh join the fray. Azula attacks her uncle and uses the resulting confusion to mount an explosive escape.
6 Vs. Aang & Katara (Crystal Catacombs)
Tired of constantly running from his own people, Zuko falls for Azula's honeyed words and fake promises about their father's willingness to love him once again. Therefore, he takes her side against Aang, Katara, and Iroh (the last of which is probably the most horrible thing he has ever done.)
While Azula fights Aang, Zuko tries to take down Katara, but neither party is in control of the outcome until the Dai Li crawl out of the caverns to lend their support. Aang is shot down, but Iroh thankfully takes the step of protecting him by making Katara flee with the Avatar's "corpse."
5 Vs. Ozai (Solar Eclipse)
Zuko never seems to run out of epiphanies, but the one he experiences in the Fire Nation secret library is his last one. He learns that his destiny, as the descendant of Avatar Roku, is to support his great-grandfather's reincarnated self against his father. During the eclipse, Ozai has holed himself deep within the bowels of his palace, waiting for the moon to finish its journey, when Zuko suddenly appears and basically renounces all association with him.
His dad taunts him into lingering just long enough for the first shard of sun to poke through, then sends a burst of lightning at him. Having finally understood the philosophical ramifications of his actions, Zuko bends the lightning back at Ozai and runs off to free Iroh.
4 Vs. Combustion Man (Western Air Temple)
Having rid himself of his daddy issues, Zuko attempts to prevent Combustion Man from destroying Aang and his friends. Strangely, the so-called assassin for hire refuses to stop, which makes absolutely no sense considering that it was Zuko who had paid him in the first place.
In any case, he fully intends on murdering the Avatar by causing the Western Air Temple to slowly collapse into the valley below, roughly pushing Zuko away when the latter physically tries to stop him. In fact, it looks like the Fire Nation prince has been thrown over the cliffside by one of the villain's attacks, but saves himself in the nick of time.
3 Vs. Azula (Conclusion)
Zuko challenges Azula for the throne, to be decided once and for all by an Agni Kai contest under the influence of Sozin's Comet. Unsurprisingly, both Firebenders are violently overpowered, producing infernos with each swing, but he gradually starts gaining ground because his sister is apparently losing her mind, consequently affecting her technique and speed.
Zuko can't exactly claim victory here, because it is Katara who takes care of Azula at the end, but the fact that he is willing to sacrifice his life for her proves that he is going to be a better ruler than most of his predecessors.
2 Vs. The Red Lotus (158 AG)
At the ripe old age of 75, Fire Lord Zuko teams up with his old friend, Sokka, now the Chief of the Southern Water Tribe, as well as Tenzin, Aang's youngest son, and Tonraq, Korra's father, to fight off the Red Lotus (composed of P'Li, Ghazan, Zaheer, and Ming-Hua.)
These criminals try to abduct the toddler Avatar for purposes that aren't quite clear at this point, but the heroes, old-ish though they are, save the day at the end. Further, Zuko and Unalaq work together to create a special cell deep within a glacier in which to imprison P'Li.
1 Vs. The Red Lotus (171 AG)
When Zaheer gains Airbending powers, he breaks out Ghazan and Ming-Hua, and all three of them speed to the North Pole to free P'Li from her ice-jail. This time Zuko is 88-years old, and well past his prime, and therefore is unable to keep up with the younger and much more nimble Earthbender, Ghazan.
He maintains his stand for a short time, proving that his Firebending hasn't rusted entirely, but soon gets thrown off guard when a boulder comes hurtling in his direction. Unfortunately, the Red Lotus succeeds in their reunion, leaving the fate of the world in Avatar Korra's hands.