The entirety of the Shadow Yosuke battle against Johnny, with Johnny using clever tactics with Tusk to try and go against Shadow Yosuke (More or less figuring out that the Persona is weak against it) including knocking it down by using the wind cyclone to lift its foot up and knock it off balance, use a low-powered cyclone to move around, and use the fact that he's fighting in a liquor store to his advantage and use some rum to set Shadow Yosuke's head on fire!
Also, the summoning of Izanagi who has now also gone through a Araki-style makeover thanks to its user, including its bandages being covered with red and blue stars and the back of its hands having white hearts on them. Not to mention the resulting beatdown it gave to Shadow Yosuke since its weakness is electricity.
Chie defeating a Shadow with no Persona and only her fighting skills. After seeing how Yosuke defeated two others similar to it, Chie was able to figure out a strategy to kill the Shadow easily.
Yosuke, using Jiraiya, beating the shit out of Chie's Shadow after shouting a hot-blooded speech in Chie's defense. Jiraiya even did a Battle Cry!
The fight with Avenging Knight. Despite the power of the Shadow being a callback to Silver Chariot, the fight shows the strengths of the IT fighting like Jo Jo characters.
Chie's fight with Shadow Yukiko is a Crowning Moment of Awesome for the Kung-Fu tomboy, who shows that while she might not be that smart academically she can think quick and kick Shadow ass even quicker.
Gyro's introduction in Chapter 16, full stop.
Despite it being suicidal and stupid, Johnny spending all night trying to ride Slow Dancer who kept dragging and deliberately injuring him, and refusing to give up despite his rather severe injuries is pretty badass.
The fights with the original Shadows, Blank Nurses and Doctor Feelgood. Both are creepy and dangerous, and require the heroes to deal with them like a legitimate threat rather than just some weak books.
Tusk evolving into Act 2, Johnny's nail bullets, and Johnny and Gyro working together to defeat Shadow Kanji.
Izanagi, Jiraiya, and Tomoe beating the ever loving shit out of Shadow Kanji. It being a smug and sadistic villain until then only adds to the satisfaction.
All of the team up attacks in Seven Nation Army Part 2 which comprise a new interpretation of Chie and Yukiko's Twin Dragon Festival, Johnny and Gyro teaming up and shooting down the hands with their respective Personas, and Kanji jumping off of an unaware Yosuke to interrupt Stormy May Day with Take-Mikazuchi and have the hands fling outwards in multiple directions at top speeds.
Also, Kanji defeating Burning Beetle with a surprise tactic and landing the final blow on Contrarian King with his move Beat It.