It can be a comics series, novel, a particular storyline, video game, or just a character. I'll start with mine.
Luke and Mara's Union comics- I'm a huge Luke and Mara shipper so these comics were like an early birthday present for me. Legends Luke and Mara are the best thing about legends for me.
A'Sharad Hett's character development- I love how we got to see a different look at the Tuskens through him. Him and his father were Jedi Tuskens who earned their respect and their loyalty until Aurra Sing changed the course of A'Sharad's life. The death of Sharad was his son's first step towards the Sith Lord he would eventually become. I also love how A' Sharad's status as a tusken brought out Anakin's secretive hatred for them and despite the fact that Anakin tried to kill A'Sharad because of it he still didn't rat him out to the Jedi. His turn to Darth Krayt and his One Sith rule has made him my second favorite Sith Lord in the entire franchise!
Legacy comics- I love how the skywalker and solo legacy has changed decades after the Yuzzhan Vong war. Cade and Ania, despite wanting to be left alone are plunged into galactic conflict and are forced to accept their heritage and their role in the galaxy's salvation.
Jacen's development in the Yuzzahan Vong conflict- I love how selfless and compassionate Jacen was despite his continued exposure to the darkside. Ignoring his eventually turn for a minute, I truly believe Jacen was the most Qui Gon Jinn like Jedi of the NJO era. He emphasized showing compassion to all life even though of the Yuzzahan Vong and he clashed with Luke to negotiate peace with them. He also expressed his desire that the jedi remained separate from the political affairs of the New Republic. He even temporarily decided to quit being a jedi when he was afraid his continued exposure to the darkside would turn him into something he doesn't want to be (ironic right). Jacen is such a phenomenal character (Before he became Darth Caedus).
Jango Fett Open Season comics- These comics are a must read for anyone who wants to learn more about Jango! I absolutely love his origin story because we get to see him as an innocent child who lost his family and found a new family with the mandalorians. He rose through the ranks of his new mandalorian family despite his very young age and we get to see how he becomes the Jango Fett we know from AOTC.
Thrawn Trilogy- The OT trio of Han, Luke, and Leia at their best, the first meeting of Luke and Mara, Thrawn at his most lethal, the birth of Jacen and Jaina. What's not to love about this trilogy?
Quinlan Vos- I love his struggles with the darkside and the fact that the very jedi who placed him within Dooku's inner circle, questioned his loyalties. He almost lost himself to the darkside so many times but his love for Kahleen and the trust of Aayla and Tholme pulled him back from the brink. Speaking of Aayla I love their master-student relationship because Quinlan was determined to be their for her despite the fact that both of them lost their memories of each other and their own identities. Quinlan just gets so much awesome development in legends in general.
Kir Kanos- Criminson Empire was a great story showing what happen to Palpatine's Royal guards following his death. I love that despite Kir's desire to exact revenge on all those he blamed for Palpatine's death he wasn't above partnering with the New Republic to satisfy his goals. He even built a rapport with a New Republic agent named Mirith Sinn. I love his conflic with Carnor Jax. It felt like the royal guard equivalent to Luke vs Vader lol.
Luke in Dark Empire- Despite my gripes with Palpatine coming back I love what it did for Luke's character as he was forced to literally resist the darksde from within and he became what he was merely pretending to be. With Leia and Han's help he was forced to realize that he couldn't do everything alone and he needs to lean on his friends and family in order to protect the galaxy from the Sith and the empire. A lesson that Mara would reinforce to him later.
Leia's mission to Tatoonie leading to her fathers origin story- Up until this point Leia never really reconciled the good man that Anakin was with the monster she knew. Despite the fact that Luke informed of the good inside him she had to experience his good nature for herself by reading up on her grandmother Shimi and talking to some of Anakin's old friends on Tatoonie. I loved watching Leia continue to show disbelief for the stories of this kindhearted and selfless child that would one day become her worst nightmare. I also loved how visting Obi Wan's home on Tatooine was the final eye opening experience she needed because even years after his death she could sense the turmoil he was forced to live with concerning Anakin's downfall. It was also a nice tibit that Leia was afraid of having children out of fear that they would turn out like her father (Hi Jacen).