Endless Legend -- picked it up mostly on sale when they released the newest expansion (though it was still $30). It has a steep learning curve, despite or perhaps because of the strong resemblance to Civ V. The uniqueness of each faction and the abbreviated (compared to say Galactic Civilizations) but powerful unit design feature really has me enthralled though.
For instance, if you like building "tall" rather than "wide" in Civ, you might like the Cultists faction, who are only allowed to have one city, but the city gets massive bonuses to food, industry, science, gold (called "dust" in the game) and influence (diplomatic currency). Using the city as a base you go around laying waste to the other factions' cities and converting unaffiliated villages to your doomsday cult. I never really liked conquest in Civ V because I always felt like I had enough to do peacefully developing my cities, but the Cultists in Endless Legend have freed me to pursue global genocide.