For SF, I'd recommend Niven (esp. when partnered with Pournelle or Gerrold), Jack Vance, John Brunner, David Weber's Honor Harringtons are a good light Space Opera read, and I'd also go with Harry Harrison, but not after the first Stars and Stripes book: my feeling is that while age hasn't diminished the man's imagination, it has played merry hell with his storytelling and his craft (for this same reason avoid The Stainless Steel Rat Sings The Blues, ...Joins The Circus and ...Goes To Hell). Also recommend early Norman Spinrad (Bug Jack Barron and The Men In The Jungle are still incredibly powerful pieces of writing). And anything at all that you can find by Walter J. Miller (A Canticle for Liebowitz for example).
Fantasy-wise, I unreservedly recommend Hope Mirrlees' Lud-in-the-Mist, and anything at all by Tim Powers except, maybe, Earthquake Weather (still recommended, just not unreservedly). Barry Hughart's Master Li Trilogy (The Bridge of Birds, The Story of the Stone and The Eight Skilled Gentlemen) is also recommended, as are any of Holdstock's Mythago Wood stories. Beyond that, most every fantasy novel seems to end up being a complete waste of time. Especiall avoid Anne Perry's 'religious fantasy' Tathea and sequel, or you could feel a strong and slightly hysterical urge to go out and nut a randomly selected religious person.. Nothing heavy, just to get your own back as it were. ('Admirably well-written' -The Guardian: pfui! Linear plot, 1-dimensional characters, and absolutely no tension or excitement or, worse still, humour. Nothing there I'd describe as 'well-written'. Unless they were talking about grammar, spelling and sentence construction...)
Crime fiction: Reed Stephens has a couple of cracking detective thrillers, The Man Who Killed His Brother and The Man Who Risked His Partner. Dashiell Hammett was incapable of writing anything put-downable at all until they jailed him for not naming names: esp. recommend The Maltese Falcon, Red Harvest, The Glass Key (3 great books about moral responsibility) and The Thin Man. Lighter in tone, anything at all by Rex Stout (he wrote far more than just Nero Wolfe), and Chester Himes is almost always good. While avoiding Robert B. Parker's later stuff, if you can forget the TV show the Spenser novels are actually quite undemandingly entertaining, until Parker had him go all James Bond/superman towards the end of the series.
Westerns: Louis L'Amour of course. Anything at all. If you find a napkin he scribbled on, go ahead and read it. The only western writer I'd really recommend. The only one you really need.
General fiction: the usual suspects, basically: Ken Kesey, Joseph Heller, Harper Lee, William Kotzwinkle (esp. The Midnight Examiner and, if you can find it -and if you do, send it to me once you're done with it- The Fan Man), and so on. Rant Alert: Avoid Martin Amis and anyone who's ever won a prestigious 'literary award': OK, there are exceptions, but most such are psuedo-intellectual semi-illiterate wanking exercises by folk incapable of telling a story. And if you're writing a novel, There Is No Other Point. (And that's not just an opinion. Writing for art's sake is the usual reason people keep telling me, "I read a book once. Took me three months. It was crap. Never again." The real reason 'artistic' writers eschew 'mere' story-telling is because It's Hard, And They're Not Up To It. End of, pardon me, story.) And even Martin Amis' dad (Kingsley Amis) thought Martin's stuff was "not good" (IIRC the quote I saw). Rant ends. Also recommend Journey To The West: the Arthur Waley abridged translation's still available from Amazon and other places, along with 2 or 3 other versions, including the 4-volume Anthony C. Yu unabridged translation with full annotation. In case anyone doesn't know, this is the Ming-dynasty Chinese novel telling the story of Monkey, Pigsy, Sandy and the monk Tripitaka's journey to India to fetch Buddhist scripture: but the journey's more important than the destination. It's a sort of Buddhist Pilgrim's Progress only hilariously (and intentionally) funny and readable, and it doesn't make you want to End It All in gloom and despair after a chapter and a bit (the way Pilgrim's Progress does).
Pulp Fiction: poss. a bit of an acquired taste, esp. the fantasy and adventure pulps, but if you like HP Lovecraft and/or Dashiell Hammett and/or Raymond Chandler and/or Robert E. Howard and/or Edgar Rice Burroughs, and so on, you'll be halfway there. The Doc Savage novels: only go there if great story-telling is far more important to you than writing craft: they are for the most part ripping yarns ("Apocalyptic Sagas' as Philip Jose Farmer called them), but technically most of them are quite literally first drafts of the stories, funnelled straight into the publisher, unpolished, from the writers' (usually Lester Dent) minds. Same with The Spider: if you read them like they're breathlessly enthusiastic novelizations of Die Hard/James Bond-style action movies, and if you like that sort of action movie, you'll love them (e.g. nazi agents infect NYC water supply with cholera, or a madman designs a chemical that eats steel, destroys the Empire State building with it, then threatens more destruction unless paid millions): if you're looking for great -or even just good- writing, you'll be driven up the wall by them. Bold Adventure are republishing them (concentrating mainly on the ones written by Norvell Paige first) and you can usually track them down in Forbidden Planet. The Shadow fares slightly better in terms of writing craft, if you can find any: tightly plotted little mysteries for the most part, with very little lurid weirdness (that was usually left to the radio show) by mystery pulp standards. Altho' they were published nearly fortnightly, Gibson still managed to give them a couple of polishes before he submitted them: story goes his housekeeper was forever having to clean blood off of his typewriter keys while he caught up on his sleep after a deadline. A lot of them used to be available for free download online, but Simon & Schuster apparently have plans for The Shadow -or so their lawyers claim- and have shut everyone down. That said, you might find paperback reprints from the 60s and 70s in second-hand bookshops. G8 And His Battle Aces: reprints easily available from the likes of Forbidden Planet, but not recommended -great imagination, but the story-telling is almost as weak as the writing, and the writing's really bad, even by adventure pulp standards. Even I find them hard going, and I love this kind of stuff (giant plague-carrying robot vampire bats dogfighting with biplanes over the WWI trenches, for example.) ! Any of Sax Rohmer's pre-1949 Fu Manchus are recommended: after that date Rohmer seems to give in (poss. distraught at the CCP victory and Jiang Jieishi's retreat to Formosa), and abruptly starts depicting Fu Manchu as the kind of 2-dimensional maniacally sadistic evil oriental supervillain, devoid of any redeeming humanity, that people generally assume him to be if they've only seen the Chris Lee and Boris Karloff movies, or read Marvel's Shang Chi: Master of Kung Fu comics in the 70s. Originally published in magazines like Collier's, so far more well-written than is typical for adventure pulps. Another untypically polished pulp writer was Brit-based Rafael Sabatini, who wrote the books every really great swashbuckler ever filmed were based on: Captain Blood, Scaramouche etc. Sabatini's stuff is currently ALL in print, and ALL of it is highly recommended. The guy wan't just a great story-teller, he was also a damned fine character-writer and a master craftsman. His stories just leap off the page and live and breathe for you. Some of his history's a bit -ehr- 'eccentric', but who the hell reads swasbucklers to learn the real history of a period? Hammett and Chandler's short stories from the pulps are nearly all currently in print, and Lovecraft's never unavailable. Another good pulp detective writer worth checking out is Cornell Woolrich (Rear Window etc.)