Few sentences in the history of Star Wars carry with it the cataclysmic gravity of Palpatine's single request in Star Wars: Revenge of the Sith: "Execute Order 66." With that, the legacy of the Jedi Order was razed until little remained other than fragments, left to be hunted down by the Inquisitors and Darth Vader himself. The Jedi Order all but collapsed afterward, left scattered and separated.
The Jedi Order was left broken as a result of Order 66. Here's everyone who perished as a result of Order 66.
Those Who Die in Revenge of the Sith
Starting with those we see die in Revenge of the Sith, we witness the death of multiple major Jedi. While we see Anakin storm the Jedi Temple, slaying a multitude of younglings, none of these characters are particularly major figures in the Jedi Order, beyond serving as the next generation. One Jedi who stood to defend the Younglings, however, is Cin Drallig, who, alongside Padawans Whie Malreaux and Bene, dueled Darth Vader. Yoda and Obi-Wan witness their fight on holographic archival footage. They did not win.
However, we do see many Jedi engaged in combat at the time of Order 66. Ki-Di-Mundi, the Jedi with that cone-shaped head who appears throughout the prequels, is shot through the stomach by the Stormtroopers in a surprise attack while storming a bridge on Mygeeto. The Twi'lek Aayla Secura is shot in the back on the forests of the outer-rim planet Felucia. Secura didn't see the attack coming because she was distracted by a bird.
Plo Koon was blown to pieces when he was flying on patrol over Cato Neimoidia. Stass Allie, situated on the desolate outer rim planet of Saleucami, was killed without warning when, again on patrol, her fellow patrolmen shot her dead.
Other Deaths
However, beyond just those who died in Order 66 in Revenge of the Sith, we later learn that many other major Jedi died off-screen as a result of the Order. Jaro Tapal is the master of Cal Kestis, the main protagonist of Star Wars Jedi: Fallen Order. Tapal and Kestis fought alongside one another during the Clone Wars. Unlike many of the aforementioned Jedi, however, Tapal and Kestis did have enough time to react to the Clone Troopers' betrayal. They were on a Star Destroyer in the Bracca System when the order was issued, which resulted in every Clone Trooper on the ship to converge on the two Jedi. While Tapal was mortally wounded, he managed to kill many Clone Troopers and damage the reactor for the Star Destroyer. He instructed Kestis with his dying breath to wait for more orders from the Jedi Temple, unaware of what was happening on Coruscant.
Star Wars Jedi: Fallen Order also features the death of Chiata, who survived a crashed Star Destroyer only for the Clone Troopers to shoot her down. Chiata's Padawan, Marseph, fought alongside her and was fatally injured in the fight. He survived long enough only to bury Chiata.
Other major deaths included Depa Billaba, Mace Windu's Padawan, who managed to have a premonition of the other Jedi dying. She, along with her apprentice Caleb Dume, fought against the Clone Troopers in one last battle. While Dume survived, Billaba died. The events occurred in the comic Star Wars: Kawan The Last Padawan. Dume, however, would later change his name to Kanan Jarrus, who fans of Star Wars Rebels would recognize as Ezra Bridger's own master. The comic Darth Vader: Dark Lord of the Sith would also feature the death of Palabee, who was lured to the Jedi Temple during Order 66 and died.
Shaak Ti, whose death was cut out from Revenge of the Sith multiple times, also dies during Order 66 in the Jedi Temple. While Shaak Ti recorded a message urging whoever saw it to not let the Jedi Purge be the end of the Jedi Order, she is still stabbed in the back by Darth Vader.
Rodian Jedi Knight Huulik, in the novel Heir to the Jedi, who was shot by Troopers flying the ship with him. However, he lived long enough to kill his attackers, record a warning to the other Jedi and land his ship on his homeworld before dying of his wounds. Visiting Huulik's remains later helped Luke learn how to construct his own lightsaber.
In addition, in Star Wars: The Clone Wars, Tiplar also died before Order 66 thanks to a malfunction in Clone Trooper Tup's inhibitor chip. He prematurely reacts to the Order 66 kill order, and shoots Tiplar in the head as a result. However, this was before Order 66 is officially issued.
No Longer Canon
Legends Canon has a multitude of additional Jedi who died as a result of Order 66. Due to the new Disney canon, these Jedi might no longer even exist, but they died all the same in the various retconned comics, novels and games.
Jedi Ali-Alann, Jedi Battlemaster Cin Drallig, Gate Master Jurokk, Serra Keto, Tru Veld, Jocasta Nu, Olana Chion and countless others were killed at the Jedi Temple, either by the 501st Legion or Darth Vader. Sian Jeisel killed herself along with many other Troopers in an explosion, saving countless Jedi students in the process. She could not save Du Mahn who died before she blew herself up.
Of all the deaths in the Legends Canon, however, the most notable is Etain Tur-Mukan, who was accidentally killed by a Padawan trying to escape Coruscant. This marks the only Jedi to die entirely by accident during Order 66.