The discovery that Saki can deliberately keep her score in ±0 in a game of mahjong. As Yuuki says, mahjong is a game that is mostly luck-based, with even professional players have an average of 30% chance of winning. A ±0 score is basically breaking even, something that is explicitly stated to be harder than actually winning. Saki does this all the time and Hisa attributes this to both incredible luck and incredible skill.
Saki again being roped into playing mahjong with the Kiyosumi Mahjong club in order to see whether or not she can still keep a ±0 score against people who already know how she plays. Nodoka decides to take the game seriously with the intent of proving that it is impossible to deliberately maintain a ±0 score (she tried doing it online, but couldn't even do it once). She declares Riichinote a player can bet 1,000 points if he/she is one tile away from winning, which then forces the player to deal all subsequent drawn tiles unless it is the winning tile, putting extra points on the table. Thus the only way for Saki to maintain a ±0 is to win via a very specific hand (70 fu 2 han). Cue Saki winning via that specific hand.
Fukuji wins her match in the Nagano finals and what makes it awesome is how she does it. Controlling the flow to prop up Yuuki in order to weaken Jun. With the power of her right eye, she negates Jun's ability to sense the flow of other players too. As the commentators pointed out, the second half of the match ended in a one-sided wipe out.
Just for comparison, she won about 7 or 8 hands in a row, a feat that is only matched or exceeded by Teru Miyanaga (whose ability revolves around serial winning) as well as the combination of Yuuki's East Round dominance and Hisa's bad luck bets. While other players have won more points than her in less hands, the sheer dominance she displayed was a record unbeaten for almost a decade of real-life time.
Kana gets several of these, namely when she gets a Yakuman hand earlier than she expected and passes it up solely because it interferes with her gameplan of getting 30 exhaustive draws in a row and one direct yakuman in order to win. And she seriously believes she can do it.
Momo getting the most points in the match against the vice-captain of Kazekoshi, Touka, and Nodoka. And all this from a character whose face had never been shown until halfway through the match.
Saki defeating Koromo by getting a Rinshan Kaihou, as well as taking advantage of a rule that forces Koromo to pay the points, which enables Saki to overtake her and win.
Achiga-hen: Nara qualifiers and National Tournament Side A
Team Achiga curbstomping all the schools in Nara, including the perennial Nara representatives Bansei high. It doesn't even take an episode for them to win the Nara qualifiers.
Season 2: National Tournament Side B Quarterfinals
Yuki's level up between the Regional finals and national qualifiers is amazing to watch. Not only is she considered THE player to watch even when Jindai Komaki (one of the only named national level monsters in season 1) is playing (granted they know to only watch her in the east round), she absolutely dominates her match UNTIL Jindai gets possessed by a god to defeat her.
, which means that the manga's universe has many queer characters in it. Furthermore, it is later revealed that Hisa Has Two Mommies, although said marriage ended in divorce.