where to start, where to start?
I am not much on the fantasy line, unless they have a good dose of humor, such as Robert Aspirin's Myth series, "Little Myth Marker", "Mything link" , etc. the notable exception being those that are on the Arthurian legend.
On older SF I particularly like: all the Heinlein novels involving Woodrow Wilson Smith, AKA Ted Bronson, lazrus Long, etc. starting with "Methuseluh's Children"; Asimov's Robot and the Foundation novels (especially the first 3), Philip Jose Farmer's novels with the real stories behind the pulp fiction heroes Tarzan, Doc Savage, Sherlock Holmes, Phinneas Fogg, etc., but not so much the riverworld series which got redundant an repetitive; Harlan Ellison as an suthor, but better still as an editor in Dangerous Visions; Samuel (Chip) Delaney; Keith Laumer ; Hary harrison (esp. the Stainless Steel Rat stories); and so many more.
A word to the wise, it was hard to make a living writing SF back when the primary sales were to the pulp magazines. the authors were paid by the word (2 to 5 cents per), so the emphasis was on turning it out quick and in quantity. A lot of the stories that were published as Ace Doubles today would have been expanded, and bulked up with intellectual steroids so that they would have been much bigger books, or trilogies or series. Some of the most successful of these early authors had many pseudonyms, so that you wouldn't feel that when you bought the current copy of Analog that you were buying a magazine written mostly by one or two authors. ;-)
Among more contemporary authors : Niven, possibly the best of "hard" science authors; David Weber whose Honor Harrington series is the SF version of the "wooden ships and iron men" novels of the like of C.S. Forester, Bernard Cornwell, etc.; Lois McMaster Bujold's Miles Vorkosigan Adventures; Robert Aspirin's "Phule's Company" series,: Spider Robinson's Callaghan's Crosstime Saloon series, and harry Turtledove's alternative history series are fun too.
Hope that helps you get a start. Welcome to SFAD!
Al (in Canada)