Hearthpwn

I made it to legend on June 14th in EU with Reno lock. I play on phone, so no stats, apologies for that.

I got 20 upvotes, so here is the card by card explanation together with matchups. Happy to answer any questions in the comments.

Class I was happiest to see on the other side -> Priest > Paladin > Warrior > Shaman > Druid > Warlock > Mage > Rogue > Hunter <- Class I hated seeing.

Always keep: Dark Peddler, Imp Gang boss, Reno

When I say keep all AOE, I mean: Doomsayer, Demonwrath, Hellfire, Shadowflame (with Panther only)

Hunter (highly unfavored): I won like 10% of my games against them. When I did, it was because they overextended into Doomsayer/Demonwrath and didn't have a deadly shot for my next threat, plus I could control the board the turn before Call of the Wild. Assume you will almost always lose, keep as much early game as possible. Use combo pieces to gain board control.

Rogue (miracle is unfavored, nzoth is favored but uncommon). Mulligan hard for premium 4 drops (Giant, Drake) and 4-dmg sources (shadow bolt, panther). Try to control the board, play Reno at around 15 life. Ragnaros is often the difference between win and loss. Against Nzoth, be conservative with the combo pieces, you will likely need it to close the game in their Nzoth turn.

Mage (tempo is slightly unfavored, didn't face a single freeze). Keep early game, including all AOE, and also keep shadow bolt and kodo (but not panther). Use combo pieces to gain board control.

Warlock (zoo is slightly - 52% - favored, mirror is close depending on tech choices). Assume zoo, keep all AOE, MCT, the true 4 drops (Drake, Vendor). Fight as hard as possible for board control, silence their 1/5, be greedy with AOE if you have Reno, and pray they don't have the nuts. Once stabilized, clear their board every turn and stop tapping if you don't have taunts to stay out of crazy Leeroy upsets. Whether you win or lose depends more on their draw than on your own (a great zoo draw beats you most times, you beat a slow one most times). Use combo pieces to gain board control.

Druid (assume tokens/yogg spells concoction, which is basically even. ). Keep all AOE, Mortal coil, big 4 drops (Drake, Giant), MCT and also silence. Try to stabilize the board as early as possible to avoid soul of the forest blowouts. Once you got the board back (the turn they nourish), overextend and push as hard as you can for a win asap. Use combo pieces to gain board control.

Against C'Thun druid, coil their 4/2, save silence for Sylvanas or klaxxi amber weaver and try to Jaraxxus if you have a safe opening as they can't beat a stream of 6/6s, and those also protect you quite well from big C'thun. Use combo pieces to gain board control.

Shaman (midrange is favored, aggro is even). Keep all AOE, MCT, Farseer, Protector, Panther, Shadow Bolt, drake and refreshment vendor, plus coil if you have a 2-power dude in hand. Lastly, keep BGH on the coin. Here, you AOE more aggressively than against Zoo or Druid, since preserving life total is more important. Be smart about using ooze (i.e. drop it naked if you need to put pressure on and a single turn of doomhammer would kill you, otherwise hold it), kill doomhammer on sight, stop tapping and taunt up once board is stable, keep clearing totems. Use combo pieces to gain board control. Sometimes they have the nuts and you lose, but overall I won about 55% of my games against aggro shaman, and close to 70% against midrange.

Warrior (pirate - slightly unfavored, tempo - favored, control - slightly favored, c'thun - even). Keep big 4 drops (Drake, Giant), Shadow bolt, Panther, kodo. Try to deny draws from acolyte with doomsayer, kodo or silence, save ooze for gorehowl against slow warriors but be happy to nab a fiery axe against the others. I am not 100% sure I was playing the MU correctly against slow warriors, where I was normally aggressively playing a Jaraxxus at the first safe opportunity. If they don't have immediate Grommash + activator or Harrison pressure you win, but it's somewhat risky. Try to keep 1-2 threats on the board, clean their board, tap as much as you can. Combo pieces should be used for high value plays, but not saved for OTK.

Paladin (Nzoth is very favored, Murlocs slightly favored, aggro is unfavored but uncommon). Keep AOE, MCT, true 4 drops (drake and vendor), ooze, coil. Mulligan for aggro even if uncommon as against control you have loads of time. If you can Sylvannas away Tirion it's ok to spend combo pieces, otherwise you hold onto them for OTK kill. Play Thaurissan only with at least one combo piece in hand, try to put pressure on your opponent but be wary of overextending into equality clears. Play Jaraxxus asap against nzoth, as they can't beat him with twisting nether, but do not play him against murlocs.

Priest (super favored). Won 9 out of 9 games played against them, felt super easy. Mulligan hard for Drake, mulligan away every other card. If you have seen at least one flash heal or at least both soulpriests, play Jaraxxus and dominate them. In this matchup, do not play Thaurissan without combo pieces in hand, and make sure to save them for OTK kill.

Card explanation:

 Mortal Coil: Filler card, but important to keep people honest by threatening at least one ping. Would never cut, but it's often mulliganed away. The only two situations where I am ok not drawing a card from it are popping a divine shield from a 4/2 or close to fatigue against warrior where I already found Jaraxxus.

Power Overwhelming: Part of the Leeroy combo, but also key interaction with Sylvanas. In aggro matchups it should be used to support board control, in control matchups it should be held.

Acidic Swamp Ooze: Never keep in opening hand, save it for Gorehowl against slow Warriors, try to play against rogue in a turn where it's awkward for them to SI:7 in response.

Dark Peddler: Too much value not to play, look for board control tools against aggro/midrange, and extra PO against combo. Shifter Zerus is playable!

Doomsayer: Awesome card, basically a time walk t2-3 against aggro, and a great card to drop after Twisting Nether to guarantee a free spot to drop Jaraxxus. How greedy you are is really an art, I am not particularly good at it and have been burned by waiting out not wanting to play it t2 against an Argent Squire, only to play it T3 and have it eaten by Flametongue totem or the like. 

Sunfury Protector: Don't be greedy against non-control warriors, it's fine to play early taunting 1 or even 0 minions.

Demonwrath: Bread and butter AOE, ok to kill 1 minion only against midrange decks, pretty bad against Shaman.

Shadow Bolt: Premium removal, kills totem golem and frothing berserker, never leave home without it.

Earthen Ring Farseer: Filler 3 drop, don't be scared to just play it to contest the board

Imp Gang Boss: Super powerful 3 drop, drop it at the earliest opportunity, not much else to say.

Jungle Panther: Most original card on the list, it's great at acting as a second shadow bolt and also works great with shadowflame as a 2-turn AOE play.

Mind Control Tech: Tech card that owns Shaman and Zoo, play it out at any other point against midrange/slow decks as it's just an overpriced body.

Hellfire: AOE plus a little bit of reach, core.

Shadowflame: AOE that works poorly against aggro, but great against control, don't keep in the mulligan unless you have panther as well.

Defender of Argus: A bit hit or miss, key post-Jaraxuss, it allows your 4 power guys to kill x/5s which is quite important against Shaman and Druid.

Refreshment Vendor: Unsure whether this or Sen'jin is better, filler 4 drop.

Twilight Drake: One of the best reasons to play control warlock decks, this card is OP.

Spellbreaker: Silence is borderline in this meta, one of the weakest cards in the deck.

Big Game Hunter: Necessary evil, feels borderline playable but you need it on the slow matchups, and it's key against shaman if they draw the 7/7.

Faceless Manipulator: Late game threat as part of combo or simply copying a big drop. Keep the combo against Nzoth decks, other than that use freely. 

Leeroy Jenkins: Worst card in the deck 95% of the time, but it wins it the other 5% of the time so it earns it spot. Can be used as removal in a pinch

Stampeding Kodo: Tech card, great against Warrior mostly. Decent body, makes Argus better. This deck really likes these body-and-removal kind of cards.

Siphon Soul: Slightly underpowered, but universal removal is playable. Crap against anything quick. Sometimes used to gain life for a bit of margin against burn end game situations (shaman, mage), even on one of your own minions

Emperor Thaurissan: Against control/combo, you MUST make sure at least one of the 3 combo pieces is in hand when you play him. Other than that, Jaraxxus is premium, rest you are just happy to have a few cards become efficient. Always assume it will get killed on sight.

Reno Jackson: Play for the body against priest, against others make sure you are healing yourself. I always keep it against non-priest opponents

Sylvanas Windrunner: Play out when there's at least 2 minions to 0 on your side against midrange, you are fine to slow game down and get a 2 for one. Against slow decks, combine with PO or shadowflame. Against C'Thun decks, best way to win is to have them play non-lethal C'Thun then steal it away, as it plays around Doomcaller.

Twisting Nether: Don't be too greedy, good opponents won't overextend. Typically seals the deal against Shaman but you can't afford to keep in mulligan.

Ragnaros the Firelord: Great against Rogue, I actually won a bunch of games cloning him with Faceless. Solid all around threat, best 8 drop available

Lord Jaraxxus: Mid-to late game closer against midrange, including the heal, the stream of infernals is unbeatable long term. Careful about Harrison, it can cost you the game. He hates Grommash with a passion

Mountain Giant: Math says you need to tap on t2 and t3 to play on t4 without coin, tap on either t2 or t3 and play a card on the other turn on the coin. Massive threat against any non super aggro deck

 Replacement advice in the comments!