Yeah, I'm going to have to go with Star Wars being Star Wars. It's pretty much timeless and kids of each generation get into it through a new iteration (See The Clone Wars TV series) or through a re-release (See the recent Blu-Ray and 3D releases) of the previous ones. Case in point, me, I got sweeped up in all the hype for The Phantom Menace as a 7 year old and watched the originals with my brother while he was in the hospital (Long story). We loved those movies and bonded over them and such.
Star Wars is a huge part of our lives like nothing else (Maybe Nintendo) and we even went and saw The Phantom Menace in 3D when it came out. Yeah, the prequels are bad, but they had the same effect on kids my age as the original trilogy did on previous generations. Case in point, I met kids in High School that preferred the prequels, yeah, that's a thing...
I do think Harry Potter comes close though. That was a big thing to a lot of kids and actually got them reading. Myself including, however, I wasn't ever really as hyped about them as Star Wars. For example, I skipped a couple in theaters, only own one crappy DVD, and have only really read each book once (Plus only really own the seventh in great condition still). Of course, that's just me and I can't speak for every kid.
I've always liked Tolkien, but the books and movies are just so slow and long, that I don't they have that same punch to kids as Star Wars (Or the aforementioned Harry Potter) does to kids. I bloody loved The Hobbit, but could never finish The Lord of the Rings (The book or the movies). I'll probably change that this year, seeing as The Hobbit: An Unexpected Journey is coming out...