I'm going to suggest (as some have before me), traveller.

It's not much of a system for optimization of characters, but it really, really has a good ship-building system that functions for everything from one man fighters to small starships in any role imaginable on up to dreadnoughts of ridiculous scale (and they have deckplans for these monstrosities, I believe the Tigress Class Dreadnought clocks in at about 33 pages of detailed diagrams).

For optimizers, most of the fun is in gear. As Telok says, the assumption is that the skills are a framework. But put together the talents and scratch for battledress, and almost anything is possible.

The system has a bunch of supplements and customization rules that make it viable for almost any sci-fi setting with a little work. Here's the current edition of the rules.