Starcraft
Starcraft is the single best RTS of all time. It's been in competitive play for more than a decade, and is played professionally in Korea.System Requirements:
Windows Vista, XP or 2000* 90 MHz Pentium or equivalent
* 16 MB of RAM
* SVGA video card that supports DirectDraw at 640 x 480 resolution, 256 colors
* 80 MB HD space
* 4X CD-ROM drive
Mac OS X 10.3.9 or higher.
* PowerMac or Compatible
* 16 MB of RAM plus Virtual Memory (32 MB RAM recommended)
* Video card that supports 640x480 display in 256 colors
* 80 MB HD space
* 4X CD-ROM drive