AI War 2. It is a strategy, 4X, tower defense light game where the AI already won the war and conquered the galaxy. You have to fight your way to the AI home world to win. The game is very well optimized and designed for multithreaded CPUs. The more threads the better. The late game can have 10s of thousands of ships on the map or more.
The 2nd game is essentially a remaster of the first so you don't need to play the 1st before the 2nd. The first game uses sprites and is single threaded so it can start lagging in the late game even with a modern processor. The 2nd game also has a lot more content compared to the first and the AI is even better.