It's a tie. In fourth grade, I would check out two Goosebumps books from the school's library and have them read by the end of the day. I could not put them down!
Having recently read the Harry Potter series again (if I can say 'again'--I'd never finished Deathly Hallows until two weeks ago), those definitely brought up a lot of memories of ... yeah, obsession. Meaning, that obsession is back. Also, I found a HP calendar from 2004 but hung it up anyway.
Those books. MY HEART.
Eggs by Jerry Spinelli. I believe I fist read this book when I was eight, maybe nine(sometime in third grade), and I have been reading that same copy at least once a year, every year, for the past eight years. It's truly an amazing book, one I'll never tire of.
My first true literary obsession was the LotR trilogy. In fourth and fifth grade, my friend and I were really into those books and would talk about them whenever we had an excuse to. No regrets.
I don't know whether many people know these books, but I loved the Sheltie books. They were about a girl called Emma and her little Shetland pony called Sheltie. I also loved The Saddle Club but the first books I became COMPLETELY obsessed with was Harry Potter but that didn't start until I was twelve. :)
Harry freaking Potter! :D
I didn't really read much before I discovered Harry Potter. They sort of got me into reading.
My first major, major obsession was Harry Potter, actually. I got the book for my sixth birthday, read it faster than anything I'd ever read before, then watched the movie in theaters about two weeks later. The next year, I dressed up as Hermione Granger for the first (but most definitely not the last!) time, and had a Harry Potter themed birthday party, thus solidifying my move into the world of nerdity.
Before that... I really liked the Magic Treehouse series in kindergarten. I wouldn't call it an "obsession," though.