Water would be inconvenient because you would have to carry water with you everywhere you went if you weren't near it and earth would be practically useless if you were ever in the desert or in the middle of the ocean or a body of water.
Fire or air can be used anywhere (except space, I guess).
Water
-Doesn't really matter about your surroundings because mainly everything is made of water. The human body, plants, animals, and even the ground itself contains it. Plus the moon gives it an extra boost of power