Every time Zero takes down a higher-level enemy is impressive, but the fight against Lord Vamdemon is especially awesome - Vamdemon has managed to blind Zero, and he's got Taichi by the shirt. Then, Zero somehow fires his V Breath Arrow at Vamdemon's face with perfect accuracy, and Taichi barely avoids getting hit, resulting in Vamdemon's defeat. In fact, Taichi had given Zero a specific order using the Digivice. Guess what Taichi's order was. "Aim for my head." Holy cow. Yes, Taichi used the sound of his voice to direct Zero's aim, and trusted him not to miss - if Zero had missed, it would've hit Taichi, and it would've killed him. If that isn't one of the most impressive examples of a powerful bond between a tamer and his Digimon in the entire series, I don't know what is.
The climax of the first Alias III battle against Piemon. After seeing just how dangerous and tricky mega levels could be to seeing it all literally be shattered by an on the spot Shmuck BaitBatman-Gambit. It also set up the famous Arkadimon scene.
The first offensive against Holy Angel Castle, leading to Leo's first on panel fight after Demon's vanguard, which included Vamdemon's left over hatred in physical form, broke through the defender's ranks and made it to the commander, who stands his ground despite the seemingly hopeless odds and leads to Lord Holy Angemon discarding his robes and donning armor.
Zero Vs. Omnimon, Both of times. Despite Zero being completely outmatched it showed the best he had was yet to come and demonstrated Taichi's ability to think ahead. Omnimon also proves to be Genre Savvy.
The entire final Zero vs. Arikadimon battle, Neo Saiba's Villainous Breakdown, and finally UlforceVeedramon Future Mode curb-stomping Daemon. That is the power of a Super Ultimate evolution.
The CGI movie Digimon X-Evolution, which could have had it's own page. After all, it's only about digimon (no humans involved), a first for the series. It has almost a crowning moment for each character, but standing out are:
The main character, who is at first Dorumon, doesn't really get badass until he reaches mega level Alphamon, the most powerful of the royal knights
Omegamon/Omnimon is basically the Terminator here, since he beats almost everyone he fights, usually other megas, and only those who ran away survived fighting him.
No mention of his battle with WarGreymon and MetalGarurumon? Aside from being well animated, it has the double advantage of being a monumental, legendary clash for the franchise featuring one of the most legendary digimon... and the two equally infamous monsters that comprise him.
But the real crowning moment is at the end, where we see how he's come full circle. After spending the majority of the movie working to wipe out the X-Antibody digimon, he accepts Dorumon's Heroic Sacrifice, becoming such a being himself and delivering an epic Shut Up, Hannibal! speech... to a character that has been entirely silent for the entire movie. Then, he delivers a final strike to Yggdrasil, saving and restoring the entire Digital World.
Dukemon/Gallantmon gets a special mention, since he is the first to question his master and (Huge spoilers ahead) Stops Omegamon from killing Dorugamon, by challeging him to a duel (obscured by clouds, sadly). Suddenly, he let's himself be impalled because, and I quote "There was something I wanted to check" while completely calm thus making Omegamon doubt his master for the first time. For itself, that would usually be enough, but he goes further: He returns from the dead stronger (and note he is the only example in the franchise who had planned it), having obtained most of the answers he wanted. Then he tells Dorugreymon how he can save the world, while casually massacring the invasion force. Then, he opens a portal to yggdrasil, thus allowing him to save the world. He then reassures him that he would stand his ground against the thousands of invaders.So not only did all of his plans succeed, singlehandedly allowing for the world to be saved, by the end of the movie he is also the only that understands why the events of the movie took place.