Hey! What's your favorite old game, as in, pre-2000?
Mine...Master's of Orion II. I still play that game time to time.

You choose a race (Human, Psilon, more I can't remember right now), each having different strengths and weaknesses.
Then, your goal is to conquer the galaxy! You can do this by either wiping out the other aliens, being elected leader, or, if you choose to put that option on, defeat the Antares!
Plasma Cannon = Pwnage.
As does the Stellar Converter.
You can colonize other planets, invade your enemies and turn their citizens loyal to you, or wipe them out and recolonize yourself, or, if you have Stellar Converter, you can simply destroy the planet.
You can even customize your ship's specifics, terraform your worlds, take over Orion (And gain the awesome 'Guardian' ship to your fleet), set up treaties, and just overall kick butt.
With all the species, ways to customize the galaxy, tech to find out, and more, this game is, to me, one of the best old games of all time.
Oh, and it's a computer game. The one I have works with Windows 95 and up.