Heinlein.. Starship Troopers, Stranger in a Strange Land, The Man Who Sold the Moon, The Moon is a Harsh Mistress, Farnhams Freehold.
Niven.. Ringworld, The ARM stories (ORGANLEGGERS!!!! LOL!), Lucifers Hammer, Footfall, and the finest pair of SF Future History stories ever written... The Mote in Gods Eye, and The Gripping Hand.
I enjoy most of the old stuff.. Harry Harrison and his Stainless Steel Rat stories were a great diversionary read; still great IMHO.
There was a series.. The Beserkers.. who wrote that?
The vintage Spider Robinson was a breath of fresh air right when SF went campy and stale. I have no idea who's good nowadys; ain't picked up a SF novel in 20 years.
So, who writes like Niven, thinks like Heinlein, and can grab your soul like the Spider could now?