The best RTS ever created.
The actual story mode is incredible, and is great for learning how to play. But, in my opinion, the real fun is online play. Seeing the strategies people come up with (the infamous "Zerg rush," for example, in which you play as the Zerg race and pump out some weak units as quickly as you possibly can, overwhelming your opponent before they have a chance to even get started) is really fun. And, of course, each strategy you come across will be subconsciously added to your arsenal.
But in the event you grow weary of actual battling, there are hundreds upon hundreds of user-made maps out there, called "use map settings" (or UMS) maps, which have entirely different goals.
Here are some examples:
Defense maps - Any map with the name "______ defense." In these games, you fend off waves of enemies (which usually don't attack, but instead run right past your defenses to try to get somewhere you don't want them to get to), building as good a defense as you can.
Madness maps - Multiplayer maps which constantly give you units. If you sit there for a minute, you'll have accumulated up to a hundred units, which you then send off to enemy bases, usually to try to destroy their one, single building, while at the same time having to deal with THEIR units.
RPG maps - Just what they sound like, some people have taken to creating entire RPGs in Starcraft, leveling up, stats, and items included. Some of these take quite a bit of time, and are quite addicting, too. I recall one that took at least six hours to complete, in total. (Luckily, you can save games and come back later.)
These are just a few of the possibilities for further fun, in the Starcraft universe. It really does have something for everyone.