Warcraft II is the game that really put Blizzard on the map.  I honestly cannot recall a PC game that I was more obsessed with than Warcraft II.  While it was not the first RTS(real-time strategy) it was one of the first and it was my first RTS.  Computers had advanced enough that made RTS games possible.  Typical Blizzard testing and retesting made this one of the best balanced RTS games and despite over a decade and a half of other RTS games this is still one of the best.  My PC at the time could not handle the game and I literally bought a better one so I could play Warcraft II.  Many a late night of playing one scenario after another or the great campaign.  Still very highly recommended.