Ringworld by Larry Niven

LOTR, The Moon Is A Harsh Mistress, Stranger in a Strange Land, The Foundation Series, The Nine Princes of Amber, The Dispossessed, Earthsea, Phillip K. Dick, The Mote in God's Eye, Ringworld, A Wrinkle in Time, dang, I swallowed this stuff when I was a kid (& beyond), the list is almost endless!!!

'Wrinkle in Time' and 'The Lion, the Witch and the Wardrobe' . Don't think so highly of them now that I recognize how theistic they are. 'Lord of the Rings' in high school which of course got me into D&D. Am so sad that Ursula K. LeGuin died. 'The Dispossessed' and 'The Left Hand of Darkness' are 2 of my favorite books regardless of genre. What she did was so revolutionary.

Fahrenheit 451 - Bradbury

The Dispossessed - LeGuin

The Moon is a Harsh Mistress - Heinlein

Dune - Herbert

Dune..

Probably a Wrinkle in Time--but I didn't know at the time it was SF.

Then probably The Stars Like Dust... and then it was off to the races.

war of the worlds had me on the edge of my seat! wowzers! I was maybe 14 or 15

The Magic Treehouse novels.

When I was in elementary school, I read "The Time Machine" by H.G. Wells. It still holds a place in my heart even though it is not my all-time favorite. Soon after that "20,000 Leagues under the Sea" caught my attention and my time. I still enjoy some of the older works along with many of the newer ones.

I loved the "Half Magic" books by Edward Eager when I was a kid.

Stranger in a Strange Land is what opened the door for me.

Anything by "The Deans of Science Fiction" Asimov, Clarke, Heinlein, Card, Haldeman, Pournell, Bradbury, Bova, Lovecraft, Chandler, and above all H.G.Wells' "The Shape of Things to Come" My library used to be the most important thing I owned, now it's my glasses.

Fantasy: Eric of Melnibone by Michael Moorcock. I didn't learn about JRR Tolkien until years later - when I was in my 20s. I've not been interested in reading any other fantasy novels after JRR Tolkien. I know there are other excellent fantasy series. I've simply been content with Tolkien lore.

Sci-fi: Arthur C Clarke's Rama novels.

My tastes have changed, though. I'm read more interested in historical fiction (e.g. Patrick O'Brian books, Wilbur Smith books) than fantasy or sci-fi genres.

"The Dark is Rising", "The Weirdstone of Brisingamen," "Lord of the Rings" (I first read it in 1975), ERB's Martian Chronicles (pretty ghastly now I look back on them, but great at age nine.) All the Earthsea stuff. MZB's "The Planet Savers". Anne McCaffery's "Dragonflight. Fredrick Pohl's "Drunkards Walk". I even managed "Do Androids Dream of Electric Sheep?" but that was actually WAY beyond my understanding at the time. These are the books for which I can actually remember the places in which I read them.

I guess there are two ways to read the question. Most folks seem to have shared what they read when they were young; for me this is hard to answer because I grew up with SF/F from a very young age (I remember my father reading LOTR, Conan, and Heinlein to me) but two that I remember really liking at a young age were Tunnel in the Sky and The Moon in a Harsh Mistress (both by Heinlein).

The other meaning of "favorite early SF/F book" is the earliest written book you liked. This is actually how I understood the question before I looked at other answers. Some HG Wells is decent, but his writing can sometimes be a bit of a slog, and many of his books haven't aged well (though if you haven't read them, I do recommend his short stories). But the early SF/F book that really blew me away (although it's more often classified as horror) is The Night Land by William Hope Hodgson in 1912. Lovecraft called it "one of the most potent pieces of macabre imagination ever written". It's long, but it's well worth the work.

Childhood's End by Clarke

I've always been a voracious reader . I remember reading my first Sci/Fi book in 7th grade , by Andrea Norton , and went through everything , she wrote that , that I could find , quickly followed by Robert Heinline , Issac Asimov , Terry Prachette , David Eddings , and others .

These were some of the ones that stuck with me from my childhood, into my teens:

The Lion, the Witch, and the Wardrobe by C.S. Lewis
The Forgotten Door by Alexander Key
A Connecticut Yankee in King Arthur's Court by Mark Twain
Lord Valentine's Castle by Robert Silverberg
Phantoms by Dean Koontz
The God's Themselves by Isaac Asimov

Something Wicked This Way Comes- Bradbury, in 6th or 7th grade, first one I remember. I was addicted to Twilight Zone as a kid, (still watch the marathons) I was 8 when the series started. This type of writing was a logical pick-up for me. I'm an avid Stephen King fan and now following his son, Joe Hill.

"The Last Question" Short story, Issac Asimov.

I remember Das Kapital and at 14 carrying around Mao's little red book. Yep, I was at that age in communist circles and those females were not messing around. Giving me an advanced education based on the sexual revolution of the 60's. Beat anyone science fiction isn't it? It was my science fiction morphing on reality, revolution was the word, meet some ladies and gentlemen fixed on making a revolutionary act daily. I lived in a different place at a different time so that I may be different based on my experience in life, do not surprises me.

Childhoods End.
Another awesome book or Stranger in a strange land..

The Castle In The Attic, A Wrinkle In Time, The Chronicles Of Narnia, The Birth Of The Firebringer series

I read The Hobbit when I was 5...finished the series by 6 (including Bored Of The RIngs...haha)....which kinda ruined the whole thing for me, cuz it feels so childlike now...wished I had read it later...it's funny what a kid can read if no one tells him he can't do it. I read a lot of miscellaneous books that I just thought were 'good' until I hit The Thomas Covenant series...

When I read Ray Bradbury's "Dandelion Wine" I became interested.....when I read Ray's "The Martian Chronicles" I was hooked......