Twilight Imperium (4th edition)
My favourite Boardgame. Basically a 4X-game in boardgame form including research, voting on laws, space and ground battles and a lot of diplomacy with your fellow players.
It takes a good while to set up, the game itself takes many hours (I once had an epic 17 hour game of TI, though that was due to some missunderstood rules and a few players who took ages for their turns. Usually it takes more around 8 hours.), the standard box layout is horrendous and you need a lot of space but it is so worth it in my view.
Maybe unsuprisingly I don't find enough players to play with nearly as often as I would like.
Nemesis
Do you like the original Alien Movie? If you answered yes and you also like boardgames then chances are good you will like Nemesis.
Not nearly as much of a commitment as TI and way more flexible with player numbers (there is even a 1 player mode but I never played that), this is probably my most played boardgame.
The player characters wake up from hypersleep and find one of their crewmates dead. The ship is still in the middle of space and there seems to be something on board.
From that premise every character has their own end goal (though you can choose between cooparative or semi-cooparative play where maybe someone secretly has the goal to kill another player or sabotage the ship)
Above all I would say nemesis is a story generator:
You barely manage to limp your half dead character into the escape pod room. Somehow everything more or less worked out. Suddenly the player you worked together with the whole game announces he takes his action to launch the last available escape pod.
One round later the fires on the ship your crew neglected for too long finally bring the ship to it's end. It breaks apart and everyone who is left on the ship including you dies.
As the game is now finished the player who betrayed you reveals he had the goal to not let your character survive and took the opportunity when it presented itself.
That's Nemesis.
Nemesis Lockdown
The sequel to Nemesis and mechanically mostly the same game. Instead of in a spaceship you are now in a research facility on Mars.
There are some improvements in the rules, some more variation and some new mechanics like managing the power of the facility as it slowly shuts down. But for the most part it is more of the same. (Which I don't mind, I like both games after all)
Freedom in the Galaxy: The Star Rebellions, 5764 AD
This game is unique on this list as I have not yet played it as of writing this. I do own it, I bought an unplayed copy last year but so far I have not yet found the time to really read through the rules and then also find someone to subject to this find another player.
But I mean a barely hidden Star Wars copyright infringement in the form of an overly complex space strategy game and with a name like this, how could I not love it? Whether I played it is unnecessary in this case.