I love the medieval era of RPGing. It's such a challenge writing in an era with so many different elements you can bring in. Language, content, all of it has so many different complexities. You can go the magic wrought, the fighting wrought, etc. It can be as realistic as you want it to be, but it can also be as far from any form of reality you can get.
Medieval for sure. I must admit that I'm getting partial to the matri rpg here which I guess falls under science fiction.
The only thing with science fiction is the use of computers to get all the answers. Unless the DM knows how to handle this the player just figure out what needs to be done easy.