Based on what I write, one might assume I'm a fan of the stereotypical spaceships and big ugly aliens science fiction, like Star Wars or Star Trek, but I have a confession to make: I don't really like either of the sci-fi giants that much. In fact, some of my favorite works are what I classify as pseudo-scientific. These are the ones that try to have as much realism as possible. The ones that use actual science to explain things. I've always been a fan of Michael Crichton's Jurassic Park books (the first movie is a classic as well, but when you get right down to it, the books are better, in my opinion) and Jules Verne. I love Jules Verne's work because of the historical aspect of them: they use legitimate scientific theories of the late 1800s. So while Journey to the Centre of the Earth may seem silly by today's standards, it's always interesting to read science fiction written before we fully understood the things they were written about. From the Earth to the Moon is also quite good. 20,000 leagues under the Sea is... Well, honestly not that great. I really loved the beginning, but it didn't hold my interest.
As for more modern science fiction, the original two Alien films are great. They have all the necessary elements for a great, tense horror movie, with the added bonus of wonderfully designed aliens. Just never watch the third or fourth ones. I also quite liked The Matrix.
The Hitchhiker's Guide to the Galaxy is and forever will be my favorite thing. As with many a good book (see Journey to the Centre of the Earth) they failed to make a good movie out of it, but the original radio series and trilogy in five parts are some of the best comedy ever written. They're great from a pure comedic standpoint, but the science fiction writing is quite good as well. Douglas Adams is my hero.
And of course, there is Doctor Who. Which I love dearly, but not as much as some people. David Tennant was awesome, and Matt Smith was great in season five of the reboot, BUT: season six was a poorly thought out mess and River Song was a bad idea. Season seven, on the other hand, was quite good. Clara is a good companion. I can't say much about the new doctor, as he is quite new, and I'm not caught up yet. I still have to see the Christmas special.
Firefly. Just... Just... Wow. Firefly. It's... Something else entirely. I can't begin to describe the wonderfully varied and well-developed characters of Firefly. Joss Whedon knows how to make a good space western. All the characters in firefly have such memorable personalities. Wash, of course, is the best out of all of them, with Jayne coming in at a close second.
Futurama. I'm a fan of science fiction comedy. It's just that one quirky genre that isn't abundantly produced, but manages to mix two of my favorite things: comedy, and space.
Anyway, that was a rather extensive list of some of the greatest works of science fiction in the last two hundred years. And that barely scratches the surface of what's out there. I just love it as a genre because of how loosely defined it is. Any category that can contain both Journey to the Centre of the Earth, a book about a German scientist discovering a passageway to the core of the globe in Iceland, and Futurama, a cartoon about an alcoholic robot, a delivery boy, and a one-eyed mutant with purple hair, is a category that you can appreciate.