A big part of what made the story so exciting is the massive decisions that are made throughout the Attack on Titan series. These choices have shaped the future of the show, letting the residents of Paradis Island understand the truth of their existence and fight for their right to survive. The most impactful decisions of the bunch are mentioned below.
There was a time when Ymir played a major role in the story, before becoming a sacrifice to the Jaw Titan instead. She was a down-on-her-luck civilian who was picked up by a bunch of delusional Subjects of Ymir who treated her like a reincarnation of their god.
After the cult is uncovered by Marleyan soldiers, Ymir decides to take the responsibility upon her shoulders. However, this backfires, with the cult members hating her and her sacrifice having no impact. It taught her that she should only look out for herself over everyone else, and instills this mentality within Krista as well.
The team of Reiner, Annie, Bertholdt, and Porco was sent over to Paradis Island to retake the Founder Titan. However, during the first few moments, Porco was eaten by a Titan who was later revealed to be Ymir.
At this point, both Bertholdt and Annie wanted to cut the mission short and head home. However, Reiner was the one who convinced them to go through with the plan, kickstarting the events of Attack on Titan.
After the death and destruction caused to their families by the Colossal and Armored Titans, the trio of Eren, Armin, and Mikasa decided to fight back. The best way to do this was by joining the military.
After a grueling few years, these three — along with a bunch of other members — managed to become the highest-ranked members of the military. The fact that almost all of these recruits decided to join the Survey Corps marked a massive shift in humanity's fight against the Titans.
After figuring out that Annie was the Female Titan, the Survey Corps decided to carry out a bold plan. They decided to trap Annie in an underground tunnel so that she couldn't transform and capture her without any casualties.
Of course, things didn't pan out like this, and Annie transformed within Stohess District itself. The collateral damage was catastrophic... but, in the end, Annie was finally abducted.
After Annie's capture, it was clear that Reiner and Bertholdt were the next two betrayers. They apparently came from the same village and fed Annie the misinformation that was altered for every group.
However, before they could be captured, both Marleyan Warriors ended up transforming in a last-ditch attempt to get the Founder Titan within Eren. It was a bold move that made for one of the best moments in Season 2.
After escaping the clutches of the invading Titans but finding themselves at the mercy of the Titans in an open field, Eren tried his hardest to transform and save his friends but failed. This led to Hannes dying at the hands of the Titan that killed his mother.
This sent Eren into a fury, who decided that he would kill the Titans even if it had to be with his own two hands. He literally implemented this advice against Dina's Titan, punching it and unwittingly activating the Coordinate.
After finding out the heinous acts his father took to secure the Founding Titan, Eren lost the will to live. He outright stated that he had no business interfering with the affairs of the royal family and gave himself up to be eaten by Historia.
However, the future Queen made the bold choice of rejecting the demands of her family and going against her father, saving Eren in the process. It was a bold move... although whether it was right or not is up for debate at this point.
After exploring the world outside the walls, Eren realized that turning the residents of Paradis Island into demons was genuinely just propaganda. At the end of the day, the one thing that really mattered to Eren was his freedom, and he decided to take it no matter what came his way.
He decided to show the world just what he was planning, with his Titan accepting Willy Tybur's declaration of war in the most overt manner possible. It was one of the most shocking moments in the entire series and paved the way for what was to come.
After seeing her closest friends being killed by Eren Yeager's destructive actions, Gabi flew into a rage. She decided to infiltrate the Survey Corps and harm their forces, even though it would've probably led to her demise.
She flew into one of the airships being flown by the Survey Corps and shot Sasha in cold blood. It was only after a long while that she truly understood the gravity of her actions.
The Rumbling was a major point of discussion over the course of Attack on Titan. After reaching the coordinate, Eren managed to convince Ymir to let go of her subservience and do what she wanted, asking her to unleash the Rumbling.
This was successful, and now the entire world outside of Paradis Island is doomed to be destroyed by the Titans. Until Part 3 comes out, the finale of this story will still be up for major speculation.