Two extraordinary audio books I've listened to in the past year: Ender's Game and Speaker for the Dead, both by Orson Scott Card.
Ender's Game is fairly well-known but well-worth its reputation, and even though it's often marketed as a YA book, adults can definitely find it enjoyable and thought-provoking. The novel concerns a young boy trained from the age of six years old in a special Battle School to someday become a great military leader to help the humans defeat the malevolent alien Buggers. But from a premise that might seem to fit in a hundred different genre novels about space combat comes a story that is deeply reflective about duty, responsibility, and leadership. Two of the main characters, Ender and his sister Valentine, have a bond that is memorable, and the book never takes the expected path.
Its sequel, Speaker for the Dead, brings out the best of what science-fiction has to offer: a chance to examine what it means to be human by telling a story where we can see humanity from the outside looking in. Two anthropologists on a distant planet are suddenly, savagely murdered by a species of intelligent beings who for decades have been peaceful and welcoming. 3,000 years after the events of Ender's Game, Ender is still a young man due to the effects of faster-than-light travel. He's become a Speaker for the Dead, an occupation which requires him to tell the truth of a man's life after his death. There's an intriguing mystery, an inquiry about what we should do to honor the lives of those who have died, and the set-up for what I'm sure are other great books in the series.
One thing I'll add is that the audio book versions of each of these is definitely worth a little extra money. Several readers are used for each book, allowing for certain voices to be used for certain characters, and the overall quality of the productions is quite high.