If I had to choose an author who has had the most influence on me: the books I like, how I write, the subsequent authors I like, the genres I read, heck my world view, it would be Robert A. Heinlein.
I've read Marquez, Machiavelli, Conrad, Camus, Hemmingway, D.H. Lawrence, Elliot, Joyce, Thomas, Shakespere, Ecco and more.
I've read Niven, Azimov, Clarke, Zelazny, Anthony, Jordan, Donaldson, Vinge, Williams, Brin, Benford, Gibson, Stirling, Piper, Tolkein, LeGuin, Lewis and more.
But Heinlein was first (or second I think I may have read the Hobbit first). And his views and outlook are interwoven in such a way that they seem to interweave into you, especially as a 12-13-14 year old boy.
There will never be another like him. Pity that.
Go read this: [linky]