I enjoy both the Ray Guns & Rocket Ships style as well as the more humanist stye of SF.
I really recommened an old series by A. Bertram Chandler centered around a character named John Grimes. Wonderful fun escapist SF.
Of course the all time great Stranger in a Strange Land should be on every SF reading list. Mote in God's Eye and other Pournelle & Niven collaborations are fun as well.
Some less well known that are just wonderful is a series of short stories about Sam Gunn by Ben Bova are just priceless. You can fidn them in a couple Omnibuss editions.
I LOVE all but one of the Callahan series by Spider Robinson. There is one in the Callahan series I found contains very, at best, questionable content and that is Callahan's Legacy. About that one novel, I say, read the reviews and know it is NOT anything a child should read. I won't go into the reasons further, just know I am very open minded about all save two topics one is rape and the other involves children...still every other book in the series is fun and silly.
I also really enjoyed The Traveler by John Twelve Hawks as well as the sequel. I am not sure where it fits in the SF arena though...it's not fantasy, it's certainly not ray guns and rocketships...
And if you like Assimov I forget the name of the short story as well as collection of novels, but he wrote a series of "Locked Room" mysteries...not really SF but just entertaining. I found it, the series was called "Tales of the Black Widowers"
http://www.fantasticfiction.co.uk/a/...k-widowers.htm
Locked room here: (
http://www.fantasticfiction.co.uk/w/...-mysteries.htm)
Those are just a few I can recall off the top of my head at 5am...