That Time She Almost Destroyed Reality
Upon her return in a tale that spans AVENGERS (1963) #311-318, Nebula set her sights on kickstarting a cosmically powerful artifact called the Infinity Union by way of an Earthling scientist's compressor machine in an attempt to attain the immense power Thanos once wielded. She pushed on even when it became clear that the compressor could wipe out Earth and much of space beyond!
Luckily, a team of Avengers survived the erasing of the universe and would have wrapped the adventure up much sooner had the Stranger not interrupted, allowing Nebula to free herself and attain god-like power. She could have done untold damage to existence had Sersi not acted quickly and decisively to remove the implant channeling her newfound might!
That Time She Saved Reality
Over the years, Nebula has proven herself a heartless killer—she destroyed Xandar and the Nova Corps at one point—but she did do one truly great thing during INFINITY GAUNTLET.
It turned out that Thanos didn't take kindly to her claiming him as kin, so, after attaining the Infinity Stones, he brought her to his side mostly-dead. Still, when the Titan was having an out-of-body experience, she managed to shamble over to his throne and slide the Gauntlet off of his hand and onto hers! She immediately undid Thanos' actions, which included snapping half of all life in reality out of existence, but didn't have enough in her to handle all that power and wound up ultimately losing the item to Adam Warlock.