Old School SciFi

As I research my final term paper, I realize how science fiction is not a modern concept. Most people think "Science Fiction" and associate it with the ScyFy channel, Star Trek, Star Wars, and aliens. But science fiction writing dates back into the 1800s. I am rereading Mary Shelley's Frankenstein for my term paper and although the language definitely gives the time period away, the story itself is timeless and indistinguishable from the science fiction films conjured in the past 50yrs. Can anyone give me more examples of early science fiction writing or maybe your favorite science fiction writing and why it's worth reading?