For me, its Aether.
![Aether :aether](https://elementscommunity.org/forum/Smileys/solosmileys/../../../images/Misc/aether18x18.png)
For starters, it is one of the best, if not THE best mono decks in the game.
But the cards in themselves are also amazing and can fit in with a lot of decks.
Dim Shield is one of the most powerful in the game when it comes to stalling.
Fractal can create large amounts of offense in an instant, and can also give you denial if you fractal something like Devourer/Pest.
Quintessence is the best card for protecting important creatures.
Lightning is by far one of the best unupped CC.
Lobo/Electro has a great ability with a low cost.
PU/TU can really turn a game around, although it is pricy.
Silence is underrated and if played right, can be extremely clutch.
![Aether :aether](https://elementscommunity.org/forum/Smileys/solosmileys/../../../images/Misc/aether18x18.png)