This is a fascinating poll, by Operation Nightfall do you mean all of Order 66 or just the genocide of the Jedi just in the Temple?

If Operation Nightfall only applies to the Jedi killed in the temple, I would go with the Sacking of Coruscant. The page linked below the poll states,

For a purge just on one planet, that's a pretty big attack.

However, if Operation Nightfall includes all of the deaths caused by Order 66, I would go with it instead if the Sacking of Coruscant. I believe all but 3 members on the Jedi Council (Vader, Yoda, and Kenobi) survived, and the Jedi were all practically extinct. The Operation Nightfall page linked also supports this, stating,

Actually the sacking of coruscant wasn't a purge, it was just taking the temple and eventually ending in a treaty which after a while gave coruscant back to the Jedi. With this taken in Operation Knightfall was way deadlier in Legends which is the only continuity where it is called Operation Kightfall killed almost 80% of the jedi order it was really deadly, but it missed alot of stonger Jedi Quinlin Vos, Shaak Ti, Obi Wan, Yoda, Galen Mareks parents, ect. This is not the deadliest Jedi Purge but it is a big one non the less.

^u just kinda bummed me out by bringing to my attention that the label “Operation Knightfall” isn’t canon anymore 😭 I would have thought for sure in some guidebook the term would have been used by now. It’s so sick.

^ Yep lol Operation Knightfall is the Legends exclusive name of this purge I actually prefer this name to Order 66 or the second Jedi Purge.

I misclicked. Even though Order 66 was a success and left only 200 survivors, it was nothing compared to the Sacking of Coruscant. Thousands, if not millions of people died.

I am going with Operation Knightfall (aka Order 66)