Compared to the other Paradox Games previously mentioned, Stellaris is unique in that it is a 4X rather than a grand strategy title. Regardless, it's a pretty fun game that's still as hard as any other Paradox title.
Stellaris also allows you to design your own ships and negotiate with alien lifeforms, who can be basically anything -- even cute kittens that want to purge your wretched form in nuclear fire.
What's not to love there?
Get Stellaris on Steam and start genociding your own adorable alien races.