Paraworld, hands down. It's an older game that works exclusively on Windows XP that never really took off. It's very similar to SC2 (look it up if you don't know how it works), but it's different in the way that you could only have 51 total units, including workers, and each unit could progress through five levels (though most start at 2-3 and can progress to 5). I personally think something like that really makes the game more interesting, because instead of simply outnumbering your opponent, you have to out-strategize him. I know there is a way to make this game work on Windows 7, because I've seen other people posting LPs of it, but my computer is so freaking stubborn.. I want to record the campaign since I never actually finished it to begin with.