Starcraft is the best real-time strategy game i have played ever. Without a doubt.
Starcraft is based in the future which is opposite to Warcraft, being more fantasy. You play as all three factions in the campaign to get the whole story and because each race is so unique, the ten or so levels you play each race for are each unique. The tilesets for level are quite well designed aswell, however, they are not a patch on the voice acting which makes the story seem so much more life-like.
Alone, the campains will take you a good 35 hours to complete but there is so much more as well. There are normal campaign maps that can be played against up to 7 computer controlled enemies. There is also a good (albeit complicated) map editor that you can create campaigns with too.
However, the real attraction comes from the multiplayer both on the internet or over a LAN connection. Online, you can use Battle.net (Blizzard's own server) to play matches with people from all round the world. If you just want to compete against people at your house though, using a LAN connection only requires one CD and it's even simpler to set up. Both are great fun though the competition online can be fierce from over 9 years of honed tactics.
Anyone who is even remotely interested in rts games should buy this as it is easy to get hold of and doesn't require a modern supercomputer to play it. In fact even if you aren't interested in rts games you should play it! This is gaming at it's best!