This is, of course, a massively popular subject within historical fiction. I haven't actually read much HF set during WWII, though, being more naturally drawn to the 19th century. And it's so huge that it's kinda hard to know where to start!
Of the handful I've read, All the Light We Cannot See by Anthony Doerr is my favourite. I also enjoyed The Nightingale by Kristin Hannah.
What novels set during WWII are your favourites?