In the beginning, Discord was a troling jerk who had to be sealed away due to his chaotic antics. Then he was sealed away again when he escaped.
Then came the time he was released in order to go through "rehabilitation". Of the Mane 6, only Fluttershy was willing to give him a chance a she was the most tolerant of him. So, she had made quite an impact on Discord, showing him the value of friendship by sticking with him when none of the others would.
Despite that, not even that stopped Discord from being tempted by Tirek and betraying the others.
After getting backstabbed by Tirek, and after Tirek blackmails Twilight Sparkle to give up her magic in exchange for her friends' lives, she does so. Twilight even demands Tirek to release Discord, whom Tirek mockingly calls him out, wondering why she would consider him a friend. After all, not only did Discord betray her and her friends to Tirek, he betrayed her entire race (before getting betrayed himself). By all rights, she should be angry and should have left him out to dry for what he did due to the betrayal, but she didn't. The fact she was probably the LEAST tolerant of Discord among the Mane 6, that's saying a lot. Ultimately, Twilight was the one who eventually changed him.
So by this analysis, who do you think had a bigger impact on Discord?
Fluttershy or Twilight Sparkle?
The most tolerant pony or the least tolerant pony?