I know I often gush about Sins of a Solar Empire, and encourage people to buy SOSE: Rebellion, but having played it a lot lately, I'm reminded that I should make a couple caveats with that.
The game I fell in love with was Sins of a Solar Empire: Trinity. Rebellion was the next (standalone) expansion to SOSE:Trinity, and while it does make some improvements, I'm here to tell you to turn off the defining feature of Rebellion: the titan ships.
Titans arguably ruin the game. They change it from a 4x RTS into a MOBA, and not a particularly balanced MOBA either. This is because each player can only build one titan, and the titans scale ridiculously with level and offensive-based titans always beat defensive/support titans. So, it becomes a formulaic race to:
1) Pick a faction that has an offensive-type Titan (IE, the TEC Rebels)
2) Research and build your titan FIRST to the detriment of all else
3) Run wild all over everything, leveling up your titan as quickly as possible, before the other players can.
Whichever player does this first/fastest, wins. That's all. It entirely removes any other strategy from viability. Fortunately, when you're setting up a game, you can turn off Titans in the setup. That makes the game 10x better.
The other thing I gotta emphasize is: if you're playing single player against computers, MAKE THE TEAMS PERMANENT (and preferably every man for himself, unless you manually set up balanced teams). As in, no making/breaking alliances during play. Because otherwise, there's nothing to prevent half the computer players from deciding to form an alliance and absolutely curbstomp you and any AIs that don't play ball with the Big AI Alliance. And it gets even worse if that alliance then allies with the pirates. Might as well stick your head in the oven because you're done.