My favorite author is Robert A. Heinlein, and I would recommend any of his books. Be advised: RAH was an old-school libertarian, and his characters relfect many of the same sentiments; "because it's always been done that way" is the worst possible excuse for a given course of action. The adult content (where it exists) isn't particularly graphic; however, he is not afraid to challenge established mores.
His stories are character-based; his self-proclaimed method of writing is : he posits a given future, introduces a character, and gets him/her into trouble.
He has a number of books targeted towards younger audiences that are all excellent and I would consider rated G/PG (Podkayne from Mars, The Menace from Earth, Have Spacesuit Will Travel, Farmer in the Sky, the Rolling Stones, Space Cadet, Between Planets)
Many of his classics are PG/PG-13, due to non-sexual adult content (Double Star (Hugo) , Starship Troopers (Hugo) , The Moon is a Harsh Mistress (Hugo), The Puppet Masters, The Door Into Summer). This is the real world; there are good guys, bad guys, indifferent guys, and sometimes it's not easy to tell who's who.
Many of his later books (Friday, The Cat Who Walked Through Walls, Time Enough for Love, Number of the Beast, To Sail Beyond the Sunset) are sufficiently explicit sexually to be considered PG-13/R. Also, any book with Lazarus Long in it (with the possible exception of Methuselah's Children) should be read with care. Lazarus is ornery, cantankerous, somewhat unpredictable, and considered by many to be a bad influence; he is easily my favorite sci-fi character.
Stranger in a Strange Land (also won the Hugo) is in a class by itself, and should not be read by anyone who isn't prepared to have every one of his/her beliefs challenged; it's as subtle as a punch in the face.