For me, the strongest influences on my writing would be Raymond Chandler, George Orwell and Mervyn Peake.
Peake was the first author whose world really drew me in, much more so than any other fantasy writer. He's also, I think, one of the great prose writers in fantasy. When I grew up, a lot of fantasy novels were very by-the-numbers, and weren't especially well written. Titus Groan was a great find (the fact I found it gathering dust at the back of a school library that looked like something from the novels may have helped).
George Orwell was the first writer who I felt was actually writing about things that seemed important to me. Here was someone who liked ordinary people, who didn't regard the English as orcs, and who seemed to be interested in almost everything. His prose is still excellent, I think, and his linking of morality, politics and good writing stays with me.
Chandler, like Peake, created his own world. Both writers are easy to parody, which I think shows how distinctive their work is (Chandler is way better than any other noir writer I've read since, even the good ones). Chandler wrote intelligent, exciting stories that were about things, and never became preachy. That's a really difficult thing to achieve. Like Orwell, his prose is clear and much cleverer than it first appears.
I'd also have to nominate John Steinbeck, largely for his Acts of King Arthur, and John le Carre, whose books I like to varying degrees, but who can be excellent. Special mention would also have to go to Ronald Searle, the illustrator, and Frank Hampson, who created Dan Dare: two great artists.