Here are mine in a particular order, I didn't pick one but two because both of them deserved it in my opinion

1.The Hobbit/The Lord of the Rings
I love this series, in particularly the Hobbit because I loved the feel and themes in the book, but the Lord of the Rings ties with it. I loved them because it's not all seriousness in it, but it still delivers a phenomenal epic, especially for its time that completely defined my favorite genre of fiction, fantasy, and literature overall.

2.Ender's Game Series
Really great sci-fi series, it didn't define the sci-fi genre in any way, but it is a very deep, and very well written science fiction series, especially when you get into some of the deeper books like The Speaker of the Dead. Each of the books really made me think for a while after completing them, well most of the main ones. (the later ones adding to the series not as much)