This is always so hard! But I'll try. One of the things making it harder now is that I've read a lot of new books I've loved in the past few years and haven't had time to reread many of them, and I usually judge favourite-book-ness partly by whether I keep rereading it... Anyway, here's one attempt at a list, as to what comes to my mind right now. The order is rather arbitrary, though number #1 was a given as soon as I started thinking about the question. This is a terribly, terribly Anglo-centric list, whoops. Also, I'm deciding to count book series as one book if they form a continuous whole and you don't get the whole story without reading each of the books. (see #2 and #3 on the list)

1. Becky Chambers: The Long Way to a Small Angry Planet
2. Ann Leckie: The Imperial Radch trilogy (Ancillary Justice, Ancillary Sword and Ancillary Mercy. I really can't pick just one because I love the whole story together, though also the individual volumes.)
3. Martha Wells: Murderbot Diaries
4. Alexandre Dumas: The Count of Monte Cristo
5. Amal El-Mohtar & Max Gladstone: This Is How You Lose the Time War

I really wanted to include something by Tove Jansson, but I couldn't decide what - her letters? One of the Moomin books? One of her adult books? Maybe I need to reread... Well, it's not like I don't have time for reading in the next few weeks.