OK, so this is a no-brainer. I don't know anyone...well maybe I do, but I can't think of anyone at the moment who has not read Tolkien's Lord of the Rings trilogy. It is a seminal part of anyone's reading experience and, if you haven't read the three (throw in four, if you count The Hobbit) volumes of this essential part of the canon, then you must go out and do so immediately. Don't rent the movies. Not that they're bad. I saw the first one, but not the second two. Read the books. They're one of the few sets I can say I've read multiple times, at least thrice. I must have read them at least twice in high school and then again in college.
I can't say anything about them that hasn't been said already. Sure, The Two Towers drags a bit, but it is nonetheless essential. Personally, I think that any list without at least one of these books on it is deeply flawed. I've grouped them together as one for the purposes of....well, because it's my list, and I can do what I want.