Well obviously if the opponent has a weakness to a particular element that is usually the best one to choose as for most powerful in genral it depends on what game you are talking about. It all depends on different factors, Charcters Magic stat what level the magic is on (7+ Ice, Ice2, Ice3 8/9/10 Blizzard, Blizzara, Blizzaga) and you also find that certain magics (non-elemental) such as Holy, Flare, Ultima are more powerful than elemetal spells overall.