I would recommend looking up the Hugo and Nebula Award Winning Novels and nominations for an absolute wealth of wonders. Also, Gollancz have published a series called 'Science Fiction Masterworks' and this is another goldmine.
My favourite books of all time are in that list - 'The Star's My Destination' and ‘The Demolished Man’ by The Grandfather - Mr Alfred Bester (I am not worthy) - widely regarded as the greatest SF books of all time by other SF authors. See where William Gibson (read Neuromancer) got the cyberpunk bug from.
Read everything by him above and also J.G. Ballard - author of 'Crash' for dystopian visions.
'Last and First Men' and 'Starmaker' by Olaf Stapledon. You read this and you'll see where Clarke, Asimov, Baxter, hell everybody got their inspiration for the big stuff.
Try '2001, 2010, 2061 and 3001' By Arthur C Clarke and 'The Foundation Trilogy' By Isaac Asimov and 'Dune Series' By Frank Herbert for more epic scale.
Try 'Rites Of Passage' by Alexei Panshin which is divinely written. What a rip-roaring ride of masterful narrative and nailbiting tension. It also raises some fascinating ideas about overpopulation and future societies.
Try 'Earth Abides' By George R Stewart - read it now, or suffer mediocrity for the rest of your life. Other fantastic pre and post holocaust novels are 'Lucifer's Hammer' and 'Footfall' By Larry Niven and Jerry Pournelle and 'Earth' By David Brin.
Try 'Canticle For Liebowitz' by Walter M Miller for a spiritual epic running through millennia.
Try 'Bug Jack Barron', 'Little Heroes' and 'The Iron Dream' by Norman Spinrad for a drug hazed headf*-k.
Try 'The Deep Range' by Arthur C. Clarke for a high pressure ocean eco-adventure.
Try 'The Forever War' and follow ups by Joe Haldeman for scale and virtuosity.
Try 'Dark Universe' By Daniel Galouye for strange delights and gradual awakenings of understanding. Lightbulbs will ping on in your brain over and over again.
Try 'Do Androids Dream of Electric Sheep' By Philip K Dick and then run out and buy Bladerunner by Ridley Scott (Ultimate Cut) immediately because it's the best SF movie ever made. Actually - read anything by this guy.
Try 'Non Stop' By Brian Aldiss - magnificent pace and adventure. What a master.
Try 'Flowers For Algernon' by Daniel Keyes for a beautiful story about a man with learning difficulties - get your hankies out though.
Try 'Red Mars', 'Green Mars' and 'Blue Mars' By Kim Stanley Robinson for an epic and technical marvel of future colonisation - NASA asks him for advice on the strength of these books.
Try 'Blood Music' By Greg Bear for a biohorror - and talking of that, 'I Am Legend' by
Richard Matheson is the best SF-horror crossover ever - beats the hell out of the pathetic recent attempt at the movie. I read on the grapevine that there was a time when Ridley Scott was going to make it during his golden SF era and sadly, I shall be haunted by the ghosts of great SF films unmade.
Try 'The Space Merchants' By Frederick Pohl and CM Kornbluth for a funny as fu*! story of future identity theft and fraud.
Try the 'Ender's Saga' starting with 'Ender's Game' and don't miss 'Speaker For The Dead' by Orson Scott Card for an examination of war ethics and recovery and healing afterward.
Try 'Startide Rising' By David Brin for a great yarn about the uplift of dolphins and great apes in to sentient beings.
Try 'No Enemy But Time' By Michael Bishop for the ultimate in time travelling yarns.
Try 'The Iron Dream' By Norman Spinrad and 'The Man In The High Castle' By Philip K Dick for an exploration of what if the Nazi's were around now.
Actually, just read the whole of the Gollancz SF Masterworks series - oh thankyou, thank you, thankyou whoever put this series together!
For Kids a fantastic intro to scifi at around 10 - 12 are 'The Children of Morrow' By H.M. Hoover and 'Dolphin Island' By Arthur C. Clarke.
Enjoy as I did and feed your brain!
Oh Bugger, I forgot Stranger In a Strange Land, Damnation Alley, Brave New World, 1984, Little Heroes, Gateway, Tau Zero, The City and The Stars, Childhood’s End and Fahrenheit 451!
I’m sure I’ll think of more as soon as I post this. Dammit Janet!