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.
In Chapter 22, in what would have been a major Tear Jerker (Taichi telling Zero to let him go and find a better Tamer), Zero instead jumps after him to save him and subsequently manages to evolve to his Perfect form, AeroVeedramon.
In Chapter 29, Leo, a Perfect level digimon at this point, sliced a Boltmon in half despite being outclassed by the latter.