Blizzard used to be known for high quality releases (up to and including Starcraft 2) and this game should be first and foremost in their showcase of achievements. It was a blend of everything they got right in all other games : great story, great mechanics, great voice acting and music, great balance, great replayability, and high modding potential. At the time it was released, there was no other game that could even hope to compete with Warcraft III, and it can still go toe-to-toe with modern ones. Out of all the games missing from GoG, if this doesn't deserve to be here as a Good Old Game, I don't know which one does.