Maeshone said:
If you can deal with not getting sequel and a horrible, horrible cliffhanger ending, then my personal favourite sci-fi game ever is called Advent Rising. It's an action game with a good story, some RPG elements in upgrading your skills with weapons and powers, and there are even some choices that have a decent effect on the story. Unfortunately it was planned as a trilogy, but didn't sell well enough to warrant sequels. The game literally ends with the words "To be continued" on a black screen, with no closure what so ever. I'm still sad about that actually... And now I have to go play it again...
That game was fun as hell. Probably the first truly buggy game I ever played and noticed (there was one particular level transition on the Xbox version that would occasionally just stop the game because it would never leave the cinematic even after the main character was no longer on the same screen), but it was really cool messing around with all of the different powers, which helped liven it up a bit considering it ripped so much else off from
Halo. Dat ending makes me sad still, too.
OT: Well, as other people have said,
Knights of the Old Republic is basically the precursor to
Mass Effect, which was essentially just
Star Wars sans Lightsabers. It uses modified
Dungeons & Dragons rulesets for combat, though, which could either add to the enjoyment or kill it completely.
Zero Escape: Virtue's Last Reward is extremely heavy in dialogue... what with being a visual novel with puzzles that are difficult as fuck to figure out... but it's only for the 3DS or PS Vita.
Binary Domain was pretty interesting for what it was. I'd say relatively dialogue-heavy considering it was just a third-person shooter.
There's always the
Metal Gear Solid franchise, as well, though the player doesn't really have any direct input on the story.
There are quite a few JRPGs that could fall about halfway into the Sci-Fi prerequisite, too, though those are often much more of a "your mileage may vary" mixed bag.
And I guess another recommendation for
Deus Ex: Human Revolution as well. Damn fine game.