Hearthpwn

This deck brought me from gold 4 to diamond 7 in under a week of casual play. It's surprisingly good! It destroys annoying res priests and quest mages (which seem to be very popular rn), and holds its own against many other decks with its high, sustained pressure.

How to play: 

Always keep the quest. And always play it turn 1 before anything else. Always. Against control, you need the extended pressure. And against aggro, you should be able to keep up and play defensively until they run out of steam, at which point you have a full hand.

Mulligan guide for going 1st:

Always keep Murloc Tidehunter, Murmy, Murloc Tidehunter, Underbelly Angler, Rockpool Hunter fish flinger.

keep Primalfin Lookout if you have any of these, throw otherwise.

Keep Soul of the Murloc only if you have tidehunter

Keep Grimscale Oracle only if you have tidecaller or tidehunter

Going 2nd: quite similar to going 1st, but with more confidence in things to keep.

Keep Oracle or Soul if you have 2 other 1-drops or tidehunter. Throw otherwise.

Keep warleader if you have both (tidehunter or angler or rockpool) and (murmy or tidecaller)

Keep Call in the Finishers always. A turn 3 coin finishers is incredibly powerful, and goes far for your quest, too. If you do, keep warleader, Gentle Megasaur or Everyfin is Awesome. Even if nothing else, you can totem on turn 2.

replacements:

Megasaur can be replaced with bloodlust. Or possibly Felfin Navigator. Murk-eye might be replaceable with leeroy. Ice fishing can be the echo murloc. All others are basically irreplaceable

Edit: replaced Ice Fishing with Fish Flinger because I realised getting a 3/2 body is far better than not. It counts 2 towards the quest anyway, but with 2 less mana