C&C Red Alert 2.
I've been addicted to Skirmishes. When I was younger I could beat a few hard enemies no problem, now I suffer against medium enemies.
It's either a race to do a tank rush or build a nuke/weather machine.
I don't know how the CPU does it but it manages to build all of these things way before me and have enough cash left over to build enough tanks to kick me when I'm down.
BTW, does anyone know if it's still possible to play the game over the internet. I have a friend who lives 300 miles away and we're desperate for a one on one.