I voted for the same three that I put as my favoured elements in my war application.
Hourglass is one of the best cards in the game, being the best and one of the only sources of pure card advantage. Ghosts are one of the most efficient creatures, Eternity can grant a near-lockdown, particularly against the dumb AI, and it has good utility cards like sundial or rewind.
Entropy has a lot of unique cards. Nova is awesome. Maxwell's and the nymph are good CC, and werewolf is an awesome creature. Butterfly effect gives it access to PC, and although it's the weakest PC card, most elements don't even have one at all. And Discord... I love Discord
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When I first started playing Elements, I thought Air was a crappy element, maybe even the worst. However, since then, I've had a lot of success with monoair in tournaments, and I have revised my opinion on Air. Eagle's Eye is incredible CC, and Sky Dragon is probably the best dragon in the game. (although light and fire also have really good ones) Wings and fog are good shields. If EE isn't enough, you have shockwave and thunderstorm for more CC, and damselflies and wyrms are also very nice creatures.
My favourite card, though, is from none of these elements. Mindgate is my favourite card, but a lot of the rest of aether isn't really that exciting to me. Aether has some very nice cards, but mindgate is the only fun one.