I am amazed by how often Robert Heinlein has come up in this thread - I have never read him. Off to the library!
In the SF/Fantasy genre, I would add that the Thomas Covenant trilogy (the 1st one!) by Steven Donaldson blew me away in my late teens. As did the Space trilogy by C.S. Lewis, and anything by Ray Bradbury.

The walls of my house would probably collapse were it not for the bookcases, so it is hard to pinpoint who "most influenced" me, but here are some authors that have not been mentioned:

G.K. Chesterton - esp. "The Man Who Was Thursday" which is a combination detective story/theological treatise/fantasy/social commentary and then some.Brilliant!The rest of his writings have the uncanny ability to make you laugh and think deeply at the same time.

Annie Dillard - there are parts of "Pilgrim at Tinker Creek" that can still give me goosebumps after multiple re-readings

Frederick Buechner - brilliant essayist and his ficion has a writing style that makes me envious.

Kathleen Norris - beautiful, thought-provoking essayist; more recent, but has influenced me recently - makes me want to be monastic.

M. Scott Peck - More wisdom than most of the rest of the "self-help" crap, "The Road Less Traveled" is a classic of common-sense psychology and spirituality.

I was a bit of a Jesus freak in high school (to say nothing of now!), so the books that influenced me were more theological or philosophical in nature. Thought I'd add my 2 cents to the mix....

Shalom,
Bo