The biggest, most infamous moment in Thanos' bloody history, the "snap" that erased half of life in the universe, never actually happened. In the original Infinity Gauntlet saga, Thanos' genocidal actions were reversed after his daughter Nebula got hold of the Gauntlet and brought back time by 24 hours, nullifying Thanos' snap. This also means that, in the entire universe, only a few species are aware of what Thanos did.
Thanos is Marvel's most iconic villain. Over decades of dishonorable career, the Mad Titan has accomplished so many nefarious deeds that it's hard to decide what was his greatest success. However, despite everything that the villain has accomplished since, Thanos will always be most remembered for killing half the universe with a simple snap of his fingers. During Infinity Gauntlet, by Jim Starlin, Ron Lim, and Josef Rubinstein, Thanos used the assembled might of the Infinity Stones to perform the only deed that he believed would please his love, Mistress Death, bringing a twisted concept of balance to the universe. Thanks to Nebula's treachery and the intervention of many heroes, Thanos' actions were reversed, along with their memory.
Only A Few In The Universe Actually Remember Thanos' Snap
A story published in the recent special Thanos: Death Notes #1 points out the often-forgotten fact that the majority of the universe is not actually aware of Thanos' most monstrous action. In "The Bar at The End of the Line", by Kyle Starks, Ron Lim, Don Ho, and Israel Silva, Thanos visits a decrepit bar in a far corner of the universe, looking for information on Nebula. When he is suddenly attacked by an armada of Docatians, a powerful species with an appetite for justice, Thanos points out that they are among the few races that actually remember him killing half the universe.
In Infinity Gauntlet #6, after the infinity snap was reversed, Thanos' narrating voice points out that most people in the universe will not remember what happened, some will have a feeling that something was amiss, and only a very few will remember in full, but the memory will prove "maddening". This established fact has been mostly ignored since then, but it is actually a pretty shocking truth. The Infinity Gauntlet story is so iconic that fans generally assume it's settled history in the Marvel Universe. However, it's actually a reversed timeline, with Thanos describing his snap in "The Bar at The End of the Line" as "attempted murder" at most.
Thanos: Death Notes #1 reminded everyone that the biggest moment in Thanos' history didn't actually happen, and few civilizations even know it could have. This means that the Mad Titan's fearsome reputation isn't even predicated on the infamous infinity snap, which makes Thanos even more worthy of the title of Marvel's greatest villain.