Master of Orion 1+2

Forge an empire in a universe where population growth is stripping away planetary resources. Colonize unknown planets and trade with other races for their knowledge.
The need for galactic expansion is critical. You must conquer alien star systems to secure the resources that will guarantee your supremacy. The ultimate goal is to defeat the evil Antarans. They lurk in the coldest reaches of space, warmed by one all-consuming passion... Revenge!

One of the best 4X games ever created
Absolutely addictive gameplay with a powerful "just one more turn" factor
Tactical ship combat, galaxy-spanning