There is a lot of science fiction out there, this guide will try to point out a few series that are worth your time because they are well-written, yet will keep you up nights turning pages. The emphasis is on relatively recent books and newer authors, with a couple lost classics thrown in for good measure.
Sure, they pay some attention to the technologies and sciences of the future, but these books are, first and foremost, page-turners with epic action and worlds-spanning adventure.
Perhaps the best modern writer of space opera is Peter Hamilton. His Pandora's Star series, starting with Pandora's Star (The Commonwealth Saga), is just plain fun adventures based around some wonderful science fiction concepts and replete with many characters, overlapping plotlines, and terrific action set-pieces. They are also a bit tighter than Hamilton's earlier books, which tended to get flabby near the end. Recently, he has a follow-on trilogy of three books, starting with The Dreaming Void (Commonwealth: The Void Trilogy) which are also really impressive, and include fantasy elements as well, though are a bit slower starting than the original two books.
And, if you like Hamilton, Ian M. Bank's standalone (large) book The Algebraist is a terrific read. His larger Culture series, starting with Consider Phlebas and including Use of Weapons is similarly excellent space opera on a grand scale, with intelligent ships with a sense of humor and giant galaxy-spanning socities.
Another fun space opera is George R. R. Martin's Tuf Voyaging, featuring a phlegmatic main character who flies around the galaxy in an ancient ship solving the problems of various planets through genetic engineering. Though Mr. Martin is better known for his epic fantasy, he wrote some wonderful, quite lyric science fiction in a very fleshed-out universe, the short stories in A Song for Lya: And Other Stories are similarly great, though a bit somber.
Sun of Suns: Book One of Virga and Queen of Candesce: Book Two of Virga are the start of a very promising space opera series that has one of the best settings in recent science fiction, a giant balloon larger than a planet in which airships fly and towns set up as wooden pinwheels cluster around hundreds of self-sustaining minature suns. The books are action-packed and invoke that "Golden Age of Science Fiction" feel, laced with some strong character development. And there are (plausible) air pirates in flying ships!
Not to everyone's taste, but worth reading if you are comfortable with the oddness of a world set in the very far future, is the series starting with The Golden Age (The Golden Age, Book 1). It is a bit sink-or-swim in throwing you into the action, so it may be a good time to use the "look inside" option provided by Amazon before buying. The books start to get a bit of an Objectivist bent towards the last in the series, for better or worse.
While not all of these series are "hard" science fiction (in the sense that technology is one of the most important aspects of the story), these series emphasize the ways in which far-future technologies might change the world.
Vernor Vinge's two part series A Fire Upon The Deep (Zones of Thought) and A Deepness in the Sky (Zones of Thought) do a great job invoking an epic galaxy with millions of species of varying capabilities, and what that means for humanity, all within a faced-paced adventure. In a similar vein, Charlie Stross' Singularity Sky and Iron Sunrise use a setting where humanity has achieved a wide variety of different technology levels, and a spy from Earth has to intervene to stop a war and find a secret weapon before it is used again. Stross's books can be a bit uneven, but are worth reading.
Alastair Reynolds is another luminary of modern hard science fiction. Many of his books are set in the same universe, and are worth reading, and all explore the concept of "deep time" - galaxy-spanning exploration at the speed of light, which means that eons pass over the course of the book. I actually like two stand-alone novels by him best, Pushing Ice is about a group of explorers who find their spaceship taken to the end of the world (and the end of time) while House of Suns is about a family of clones, sort of, who travel through the galaxy, meeting every couple hundred thousand years.
If you want something less on the spaceships and exploration side, you might consider Evolution, which dramatizes the story of evolution of life on Earth from the distant past to the far future through a series of vignettes, and manages to be strangely moving to boot. Its not part of a series, but Baxter has a variety of good hard science fiction novels if you enjoy this one.
Finally, in the psychological and metaphysical vein, Blindsight is a story of first contact, and a spaceship containing various cognitive oddballs (and, effectively, a vampire) that are sent to speak with a mysterious alien presence. Dark, interesting, and challenging, while still being readable.
If you like military science fiction, you have probably heard a lot about Scalzi's Old Man's War and its sequel, The Ghost Brigades (A Sci Fi Essential Book). They are indeed quite good takes on Heinlein's Starship Troopers, but, at least in my opinion, are not as terrific as they are often hailed to be.
Another interesting take on military science fiction is Louis McMaster Bujold's Miles Vorkosigan series, which begins with Cordelia's Honor (Vorkosigan Saga Omnibus: Shards of Honor / Barrayar) but really gets underway with The Warrior's Apprentice. The series, following the story of a crippled Miles, son of an important nobleman, as he grows up on a planet with a strong warrior ethos, is much better than the short description I just gave makes it sound. While there is plenty of combat (mostly in the first four or five books), it has a different feel than most military science fiction, concentrating more on character development, politics, and plenty of banter. And, though the series has its somber moments, it generally races along and features a wonderful cast of characters.
Fantastic Science Fiction While all of these books are science fiction, they often bring in more fantastic elements.
Dan Simmons is a master of this field. Two series: Hyperion (Hyperion Cantos) and Ilium are deeply challenging, but engaging, works set in the very far future. The first series uses Canterbury Tales as inspiration, the second the Illiad.
Viriconium, a recent collection of novellas by M. John Harrison, has a similar feel, though it is less character-driven than Wolfe's classic, and, as such, is ultimately less satisfying (Harrison's Light, by the way, is one of those odd science fiction novels much-praised by critics from the New York Times to the Guardian, but often disliked by readers of science fiction - I thought it was occasionally brilliant, but not particularly enjoyable.)
Another contender among the greatest science fiction writers is Cordwainer Smith, actually a pseudonym for Paul Linebarger. Linebarger was a fascinating character: the godson of Sun Yat-sen, the founder of the Army's first psychological warfare division, and an adviser to President Kennedy. His highly lyrical The Rediscovery of Man: The Complete Short Science Fiction of Cordwainer Smith and Norstrilia view the future as something closer to myth, with bizarrely named characters and sheep that grant immortality. They are also a lot of fun to read, and very different than most science fiction out there.
Though not as beautiful a writer as Smith, China Mieville, the leader of the so-called "New Weird" movement in science fiction has an excellent and imaginative series starting with Perdido Street Station and The Scar. They read like a fantastic steampunk version of London, but are more science fiction than fantasy.
Also pushing the boundaries of space opera by crossing it with the fantastic is Richard Garfinkle's Celestial Matters. It is based on the premise that the Greece of Alexander the Great never fell, and, further, that classic Greek science was right - the planets are attached to crystal spheres, and there are only four elements, and so on. The result is the first Ptolemaic hard science fiction novel as the Greeks launch a spaceship to retrieve the ultimate weapon: a piece of the sun.
This is what I call the category of science fiction that consists of those novels that take today's world (or the near future) and add a science fiction twist, showing us a world where much vaster forces are at play than we could know - think the Da Vinci Code, but science fiction, rather than religious mystery.
The master of the genre is Tim Powers, an incredible writer who spans the boundaries of fantasy and science fiction. In his most accessible novel, The Anubis Gates (Ace Science Fiction) he weaves together time travel and ancient Egyptian conspiracies, while his recent Declare rewrites the spy novel and gives us an entirely different history of the Cold War. Charlie Stross's The Atrocity Archives (Laundry Files) and The Jennifer Morgue provide a humorous take on the same sort of world, where the hero is an IT guy and superspy working for a secret organization to stop mysterious (but grounded in physics) forces from taking over our world. Things That Never Were: Fantasies, Lunacies & Entertaining Lies is a strange little book that offers short plotlines, some fantasy, some science fiction, along the "secret history" line, while not going as far as actually providing any stories. Read it if you like the odd, otherwise, skip it.
And if you haven't read Lem's very funny and clever The Cyberiad, you really, really ought to buy it now, as he tells the stories of robot constructors who travel the universe inventing things like the a poetry machine that can outdo all other poets. It is challenged to "compose a poem about a haircut! But lofty, noble, tragic, timeless, full of love, treachery, retribution, quiet heroism and in the face of certain doom! Six lines, cleverly rhymed, and every word beginning with the letter s!!" The response from the robot begins "Seduced, shaggy Samson snored/She scissored short. Sorely shorn.."