There are a lot of good space strategy games out there; Steam is a great place to start looking.  Some titles I would recommend are "Sins of a Solar Empire: Rebellion", and the remastered release of "Homeworld 2".

Sins does not have a single-player campaign, but is instead designed with multiplayer in mind, or very long single-player matches in a "scenario" of your own design.  Sins is also highly mod-able, with some excellent Star Trek, Star Wars, and other mods from popular sci-fi universes.  Take a look through ModDB to see the nice selection of mods for this game…"Star Trek: Armada 3" is a mod that could very well be mistaken for a professional, commercial-release game in its own right.  Amazingly well-done.

Homeworld was an excellent space-strategy game from yesteryear, the first of its kind to offer a fully 3D gameplay environment (you can move ships in the Z-axis, to strike from above or below your enemies).  In addition, Homeworld also offers an incredibly rich campaign storyline, with absolutely beautiful music and voice-acting, to pull you into the plight of the Higaaran race like you'd never imagined.  Don't be surprised if some parts of the story actually tug a tear from the corner of your eye.  The original game is quite old now by today's standards, but there are some mods that brought it more in-line with current technology.  However, the remastered release completely overhauls all the visuals, in stunning HD glory, as well as making all the audio even better than it already was.  They even got the original voice actors to re-record most of the dialogue, and added some new lines.  The only reason to use mods with Homeworld now, is to change the game mechanics for single-player skirmishes.  But more than anything else, I simply recommend playing through the campaign.  It's an experience you won't regret.