Probably my most memorable books were the WoT series by Robert Jordan, simply because he started writing the books way back in like 1985 and I started reading them sometime in high school. I have literally put a decade and more of my life into these books. I usually try to re-read the series just prior to a new release. Now with his passing, I may never know how he intended the series to end.
It's certainly a set of books that I will remember well into old age.
I would also have to say tLotR by Tolkien. I realize this is possibly a generic answer for a lot of people, but for me, the reason is this... I can remember as a young child, my parents would take us to a local video store called Corenos. We weren't exactly wealthy, and Corenos would give 2 free rentals for a report card with all A's and B's. We used to get report cards quarterly back then, and so I always tried to do my best, so I could get the free movie and game rentals.
One of the earliest cartoons I picked out just happened to be "The Hobbit" as put forth by Warner Bros. The cartoon stuck into my head well into middle school, when I finally realized that the cartoon I remember as a younger child was actually based on a book. In 7th grade, I finally read "the Hobbit" as my school read it together as a class. Unlike a lot of the students in the class, I enjoyed it so much I went on to read the whole series on my own.
For a few years I quit reading after that, until a copy of "Sword of Shannara" by Terry Brooks showed up in my Grandmother's attic. I read that book, and have been reading probably a new fantasy book weekly, ever since.
Mathius