Any time Reki makes a truly ridiculous shot with her sniper rifle. For example (also the first and possibly least impressive), she shoots a bomb off the underside of a bus from a helicopter. In the anime, the excellent music makes this even more awesome.
The climaxes of each volume qualify, with the opponents getting more and more powerful and the battles higher and higher in tension and epicness.
Anime
In Episode 1, Hysteria!Kinji shoots 6 bullets into 6 different Uzi barrels, detonating them. Even Aria is left fascinated at this.
Also from the first episode, Aria's and Kinji's brief scuffle after she accuses him of ripping open her shirt (also because he mistook Aria for a junior high student due to her petite body) stands out for showing how badass and well trained are both. He locks her shooting arm in a Judo-stylekannuki gatame, which she counters with a pretty impressive harai goshi, only for him to roll harmlessly out of the throw. He then proceeds to snatch her magazine, leaving her mortified.
When Reki pulls her Big Damn Heroes moment in Episode 3 by saving the students on the bus by shooting a bullet through the guard rails on the bridge, while the bus was moving, into the small bomb strapped to the bottom of the bus while in a blackhawk flying feet above the water.
Riko being revealed as a master schemer. She did the Butei Killer attacks only to make Kinji and Aria meet and become partners, which they did, only in order for Riko to face and defeat a full-power Holmes descendant along with her Watsonian partner. The last part of her plan, when she seduces Kinji so he enters Hysteria mode and deduces Aria's plane will be attacked by the Butei Killer, making him aboard the plane so they will be together to face Riko, is equally brilliant.
Riko's and Aria's flawless Gun Fu duel in the plane, followed by Riko unveiling her psychic powers.
Rest of the series
Kinji managing to defeat his brother in Volume 4.
Volume 5 opens with one of these, a painfully epic battle between Kinji and Kinichi against Sherlock Holmes.
