Hearthpwn

This is the deck I've used from Rank 15 to Rank 5 (I always stop at Rank 5, because i've got no time/patience to grind through Legend. lol)

I think my win-rate with this deck is around 75-80%. No percentage/stats screenshots, because I've accidentally reset-ed my decktracker stats :P

Replacements:

Golakka Crawler> into Rat Pack

It's the only card replaceable IMO. The main reason I'm using Golakka Crawler is due to it's very decent/vanilla-ish body. I don't really mind placing this on board if I'm facing a non-pirate deck, as the stats is still pretty good. And If you manage to snag a pirate with it in the early phase, it will snowball. Rank 10- Rank 5, from my experience, is quite plagued by Quest Rouges with Pirates,Pirate Warriors,Aggro Druid with Pirates.

As for the Rat Pack, it has a pretty good deathrattle. My only problem with it, is it's mana cost. I find it quite 'steep'. Still a pretty good card, especially if you can buff the Rat Pack or the 1/1 Rats.

 Mulligans:

Generally, any of these, though prioritize the 1 drops:

Alleycat  , Fiery Bat , Jeweled Macaw -> Crackling Razormaw , Scavenging Hyena, Kindly Grandmother

Keep the Golakka Crawler if you are facing warrior, druid and rouge. It doesn't matter if they play pirate or not. The stats are still good. And there's no way you would be able to tell, unless you started playing. So BETTER keep this card in your hand. You can mulligan this out if you want if you are facing other classes.

As for the Kindly Grandmother - I always mulligan this OUT if i'm facing Priest, and only Priest, due to their potential Potion of Madness. The moment they get this through that card, you'll eventually lose or comeback will be very hard/seemingly impossible. The tempo swing into their favor is nuts.

Good luck and have fun hunting. And kindly upvote ;)