I concur with many of the previous speakers… but i racked my brains for a bit and came up with a list, none of which (I think) have been mentioned yet, all of which (I think) would repay reading:-
Olaf Stapledon’s Star Maker. One of the first things that got me out of the Asimov/Clarke/“Doc” Smith rut, and an exercise in Thinking Big. About as big as you can get, in fact.
Greg Egan’s Permutation City, for cutting-edge computers/mathematical speculation.
SF needn’t be serious. Candidates for best rampantly silly SF novel include Robert Sheckley’s Dimension of Miracles and John Sladek’s The Reproductive System.
Nobody’s yet mentioned Iain (M.) Banks? Use of Weapons is my favourite of the Culture novels.
John Wyndham’s The Day of the Triffids effectively defines the peculiarly (mostly) British sub-genre called the “cosy catastrophe”. And it’s a fun book.
More modern British authors include Jeff Noon (Vurt) and Geoff Ryman(The Child Garden).
Less modern British authors include Brian Aldiss (short story collection The Saliva Tree is my personal favourite) and J. G. Ballard (again, head for the short story collection Vermilion Sands).
So that’s a list from me. Some of them an acquired taste, and mostly British - not that I have anything against American SF, it’s just that other people have already recommended most of my American favourites.