Best reward card

Venari Wavesmith is without a doubt the best reward card

He's curretly being sold for 1,70$ with prices declining.
But anything around 2$ is worth it for him in my opinion.

Magic Water deck is arguably one of the best decks in bronze/low silver.
It's strong against almost anything and even counters Mylor Crowling, which is the best deck at lower ranks.

It's only weakness is Fire decks with enough speed to kill your backline, before you get a chance to attack.
Venari Wavesmith is the answer for that problem, since he gives everyone on your team +2 armor to survive that initial attack.

Honestly, if you want the easiest deck, just go with Alric, Tank in front, as many magic monsters as you can fit in the back and then Wavesmith. Best 2$ you can invest right now in tearms of helping you win games.