I voted aether because Mindgate is so fun :p I don't think aether is the strongest element, but there is just something so cool about taking your opponent's deck, making several copies of every card they draw, and then smashing their face with their own strategy.
If I were voting for the most powerful element, rather than my favourite, I'm not sure which I'd pick. Time is very strong; Eternity and Hourglass are two of the best cards in the game. Time also has one of the best nymphs and several quality creatures. Gravity is very powerful as well, with lots of momentum, black hole, several forms of creature control, (gravity pull, otyugh) and one of the best shields (Gravity Shield) and far and away the best nymph in the game. Entropy/Fire/Darkness (and pulverizer) can destroy permanents, and that alone makes them very strong.
Now that I am trying to decide, I'm not sure which element is the most powerful :p I could point to a few that are at the bottom of the heap, but I can't decide which is the best element.