It figures we have such a cool contest when I have absolutely zero chance to MoC

Anyway, I had a few ideas, and since I have no way to enter, feel free to steal.

Lightsaber: Anything from the size of the prop in the movie to the size of the plastic toy ones they sell. Blade optional, but it would be cool it you could pull it off.

Headcrab: Half-life is sci-fi enough, fairly easy shape and it works in several colors

SSV Normandy: With Mass Effect 2 on several BZpers minds I imagine there will be quite a few mocs from there, why not stand out?

Generic astronaut: not many people will do this I think, but the generic modern spacesuit appears in several "hard" sci-fi works

Brotherhood of Steel member: Cool armor, shape recognition. Some may argue that fallout isn't sci-fi though.

Any F-zero car: pleanty to work with, there were at least 40 cars in gamecube game alone

Bender: Difficult, but the result may be worth it

Rosie (the maid robot from The Jetsons): Appeal to nostalgia or something like that

In general I'd steer away from mechs or other robots, everyone and their mother will do those. Aliens are a difficult case, if you really want to do an alien, I'd go with something crazy like the Hanarr from Mass Effect. Spaceships are a tough call too. Generally I'd say that bigger is better. In terms of size of the original, not size of the model. Try building a small-scale frigate instead of a starfighter. if you're feeling ambitious I'd go for an all out space station.

Anyway, good luck to all who enter, I wish I could join you.

"No tests on species with members capable of calculus. Simple rule. Never broke it."

~Mordin Solus, Mass Effect 2