As opposed to 8 years ago when Oliver Queen was the only vigilante around, the Arrowverse is now teeming with heroes and, as more shows get made, that likely isn't going to change soon. Unfortunately, heroism can sometimes mean sacrifice and there is no shortage of heroes that have sacrificed themselves for the greater good on the shows.
Almost every Arrowverse character has died, the majority of which were undone due to time travel or supernatural means. While each character's death was meaningful and left a major impression, some deaths have been more important than others.
Roy
While lots of the characters' death are highlighted in the shows, the death of Roy Harper aka Arsenal was handled a little differently.
In season 7, when he returned to help the team deal with Emiko, he revealed to Oliver that while he, Thea and Nyssa went to go and destroy the Lazarus pits, he was killed by a member of the Thanatos Guild and was then revived by the Lazarus Pit. Unfortunately, his death didn't hold a lot of emotional weight since it occurred off-screen and didn't last.
Ronnie
As a result of an unstable wormhole being created by Barry going back in time and then Reverse-Flash attempting to use it to get back to his timeline, a singularity opened above Central City.
In an effort to stop it from destroying the world, Firestorm, who at the time was Ronnie Raymond and Martin Stein, flew into its eye and separated from each other. Unfortunately, it resulted in Ronnie dying while Stein ended up surviving the incident.
Behrad
Season 4 introduced a living version of Zari's brother, Behrad, with him replacing his sister in the timeline and taking her spot as a member of the team. However, in season 5, when Atropos boarded the Waverider in search of the other pieces of the Loom of Fate, Behrad steps in to stop her and protect his sister who was in the air totem.
Atropos then killed him by cutting his life thread after sensing that he was from an alternate timeline. His death devastated Zari but he was brought after the Legends regained control of the Loom.
Nora
Nora West-Allen was daughter of Barry and Iris who traveled to the past from the year 2049 to meet her dad. She was erased from existence when she and Team Flash worked together to destroy Cicada's dagger.
Despite knowing that it could save her life, she chose not to enter the negative Speed Force due to her realizing it was corrupting her and making her more like the Reverse-Flash.
Sara
Sara Lance aka the White Canary has probably died the most out of any character in the Arrowverse. She was believed to have gone down with the Queen's Gambit but her first real death was at the hands of Thea while she was being influenced by Malcolm Merlyn. She was later revived with the Lazarus pit which resulted in her suffering from bloodlust.
She then died again after being killed by a brainwashed Rip and then was revived by Gideon. Her most recent death happened in season 5 when she was attacked by a zombie horde with the rest of the team but was ultimately brought back by Charlie.
H.R.
H.R. Wells from Earth-19 was the 3rd Wells to be a part of the team in season 3. The entire season was built around the team working on preventing the death of Iris at the hands of Savitar, which Barry saw when he traveled to the future.
In the end, H.R. ended up swapping places with her using a device that can change the appearance of the . Even though his sacrifice was sad, especially considering how poorly he was treated, it was undercut by the fact that no one expected them to actually kill off Iris.
Shado
Shado was the daughter of Yao Fei, Oliver's mentor on the Lian Yu, who was in a romantic relationship with Oliver and was killed by Anthony Ivo when he forced Oliver to choose between her and Sara.
It was her death that led Slade Wilson to go off the deep end and become a villain, blaming Oliver for his choice and resulting in him coming to Star City and fighting Oliver as revenge.
Snart
Although Leonard Snart aka Captain Cold wasn't a hero when he ed the Legends, he certainly died as one. Snart sacrificed himself in order to destroy the Oculus and prevent the actions of the team from being controlled, allowing them to have a shot at defeating Vandal Savage.
In the explosion, he manages to take out the Vanishing point and kill Time Master Druce with his final words being "There are no strings on me".
Rip
In an effort to buy the Legends more time to get away from the time demon Mallus, Rip removed with the Waverider's time drive and planned to use it as a bomb.
Knowing that it would only temporarily delay the demon and risk his life, Sara tries to convince him not to go through with it but he reveals his desire to be reunited with his wife and son. It's an extremely touching scene in a show that has become known for its silliness.
Stein
Martin Stein was one half of Firestorm with the original other half being Ronnie Raymond who would later be replaced with Jefferson "Jax" Jackson. His death came during the Crisis on Earth-X event when he was shot by a Nazi while attempting to make his way to open a breach.
He was then taken back to the Waverider where he would later succumb to his injuries after breaking the link with Jax which was the only thing keeping him alive.