It surprises me that this game has not ever been suggested. I personally would not call it "One" Game because it has this map editor which makes everyone to create different maps, which usually are similiar to custom game modes.

The Original Warcraft 3 game is a strategy game, you build a base and eliminate the everyone that oppoeses you.

The games names are:
Warcraft 3: Reign of Chaos (2002)
Warcraft 3: The Frozen Throne (Expansion) (2003)
NOTE: The expansion pack is highly recommended to get.

Game Requirement

• 400 MHz Pentium II or equivalent, or a 400 MHz G3 processor or better.
  
• Windows 98, Windows ME, Windows 2000, or Windows XP. Warcraft III requires Mac OS® 9.0 or higher, or Mac OS X® 10.1.3 or higher
  
• Warcraft III requires 128 MB of RAM. Virtual Memory should be
  enabled on computers running pre-Mac OS X versions of the Mac OS®.
  
• A keyboard and mouse are required.
  
• A hard drive with at least 550 MB of free space
  

Max Players: 12
Activity: 9th on X-Fire weekly top list (Highly Active)

Cost: I'd guess about 5-30$ depending on the store,


But these other maps I spoke of, I'll give you some examples here:

DoTA (Defence of the Ancients): (The most famous one) Two teams 5v5, two bases (Sentinel & Scourge)
You must destory the enemy base, both bases sends equalliy strong "creeps" to attack the enemy base. Then each player gets a set of heroes to choose from and then uses this hero to kill creeps and enemy heroes that are trying to destory their base getting experience and gold. Then use the gold to buy items to become strongest.
DoTA webpage

Vamprisim: 12 Players, 10 Human & 2 Main Vampires. The human team shall run and build up a camp to kill the vampires, they are very strong and you need a though base to kill them. (Base mostly persists of towers). If a Human is killed he becomes a Vampire, which are weeker than the 2 main vampires.

The game can really not be seen as a strategic game if you play certain custom maps. DoTA is today more known as an own game and several new games have been released to act as a better version of it, such as "League of Legends" & "Heroes of Newerth"

This is one of those games you can just play forever. There is always new maps, and I doubt it would die out soon.

And consedering that the ability to learn the game depends of map just made it better.

WC3 +TFT are Old school but very good

I have Warcraft 3, but I don't have any of the expansions . Really fun game though, but didn't like the campagn very much...