I always loved Warcraft 3 TFT's world editor because it was really powerful and easy to learn. The ability to change unit names, stats, model, responses, weapon type, moving speed, abilities, nature (flesh/mechanic/magic/ethereal), etc. with but a click was an amazing achievement for many people and the why even today the game lives on with DotA (which second version is coming soon, to retire the War3's one) and other custom maps. BTW Starcraft 2 brings an even more powerful editor, but considerably harder to learn, too.