This is my life

The Vorkosigan Saga by Lois McMaster Bujold, in particular 'Memory', which taught me more about how to recover from grave, life-changing mistakes in a way where I can live with myself afterwards than most books more explicitly about ethics and morality that I have encountered

Seconding the strong rec for the Ancillary Justice trilogy, which may have sprouted from a seed of 'but what if Octavian/Augustus didn't have to find a worthy successor but could keep being emperor forever?? IN SPACE' but has far, far surpassed that. (See also bironic's extraordinary Ancillary Justice fan book trailer.)

I'm really surprised not to see anything by Octavia Butler on the list already - I would put her books as having a really strong, lasting impact (with a major resurgence right now, in fact!) in a way that's now eclipsing works like 'Ender's Game'. Hard to pick just one, but I've been thinking about 'Parable of the Sower' a lot lately, as well as 'Kindred'.

The Maisie Dobbs series by Jacqueline Winspear is a fictional mystery series set between WWI and WWII in Britain (and eventually further flung locals) featuring a psychologist and investigator who also happens to be a young woman carrying her share of war trauma who goes about not only solving but helping the community resolve tragedies that are often rooted in past war trauma. The research is impeccable (the author grew up in a family heavily impacted by both world wars), the characters change and grow over time in very compelling ways, and I absolutely adored the voice: the author has the kind of insight into how humans tick and what can heal them that befits a character of Maisie Dobbs' skills, and it's a genuine pleasure to share in that POV and learn from it over the course of the series. There are also fantastic audiobooks available, and the cover art is by an engraver who really knows his craft, so both physical and digital variants are a delight.

ETA: also seconding Le Guin, DWJ, and Dorothy Sayers! I've been meaning to do a full Le Guin reread soon, actually - I read (and reread) all of her works relatively young, and I'm very curious how they'll land for me as an adult. They've been so formative that it feels a little peculiar to think there are people who have never read her works yet?