Death Star or Star Forge?
I'm gonna have to go with the Star Forge, I mean sure the Death Star could destroy a planet...but the Star Forge created an endless supply of ships and was the very fuel any great galactic fleet. Whoever used it had a limitless fleet at their disposal...and the fact that the Sith were in control of that station makes it a bigger threat then the Death Star, if not the biggest threat the galaxy's ever faced. Well, I'm not bringing the Sun Crusher into this b/c that’s pretty damn impressive to be able to destroy an entire system, the Death Star would seem insignificant compared to it.