Hearthpwn

GENERAL MULLIGAN (Gen. Mul. for future reference)
In order to make it situation-based, I will write a normal mulligan and after it based on the two kind of decks that are currently going around. This is what you generally want to go for: 
(With or without coin it's the same) Look aggresively for cheap minions like Voidwalker, Flame Imp, Knife Juggler, Dark Peddler. 


Combos that should ALWAYS be left with the coin are Nerubian Egg + Abusive Sergeant/Power Overwhelming + Void Terror or in other case stick to the Gen. Mul.
Leave the Power Overwhelming with the Nerubian Egg ONLY if you have the Coin. If you don't, mulligan them for something better/more stabile. 

Without the coin just try to get a curve in your hand aka a card for every turn, in best case (let's say for all classes) it would be something like T1 Flame Imp, T2 Nerubian Egg T3 Void Terror T4 Voidcaller. Anything else with these values is also fine. 

I will split the other mulligans based on control or aggro decks: 

Control/Midrange Decks (Control Warrior, Dragon Paladin, Tempo Mage, Midrange Hunter, Control Priest, Oil Rogue, Combo Druid, Freeze Mage, Handlock): if you get Gen. Mul. perfect, if not also keep a Nerubian Egg, Haunted Creeper, Abusive Sergeant, Knife Juggler, Imp Gang Boss. 

If you have the Coin you can keep the Haunted Creeper and a Knife Juggler as play T1 Coin + Creeper, turn two Knife Juggler if you can feel that he will get something strong out that can kill the creeper if you hit it. Dead Creeper with a Juggler on the board = 2 free damage.

Aggro decks (Face Hunter, Mech Mage/Warlock/Shaman, Face/Ebola Paladin, Zoo Warlock): Aggresive Mulligan for Gen. Mul. especially forVoidwalker, a very crucial card against an aggro deck. 

If you can feel it's a Face Hunter, you can keep the Haunted Creeper as well. 3 easy damage to those charging motherfuckers. If you have a coin you can keep the Defender of Argus as well! 

This Midrange Zoo deck has very high potency, and if played out properly, can crush any kind of deck. If you have ever played a Zoo deck you know approximately how it works, try to overflood the board as soon as possible and that is the most important thing for this deck, even though it doesn't have that low of a curve as a regular zoo deck. The point with this deck is to try and use every single mana point to put out as much as you can. One more very important notice is to know where you are planting your cards on the board as you can always expect a Defender of Argus or Void Terror incoming. I like to keep my Deathrattle on the right side and then put a Defender of Argus in between. 

Staying on the curve with this deck is extremely important, and it can bring you some sick plays, like a Voidcaller suicide/remove into a free Doomguard/Mal'Ganis or a Knife Juggler into an Imp-losion or a Voidcaller on the board removing a minion then eating the Voidcaller with Void Terror and getting a free Doomguard out e.g. Your early game will be just like any other Zoo deck, try to get at least one creep out, NEVER tap over playing ANY of the minions, even if you are missing one card for an amazing combo! Your Tap will slow your game a lot and will put your opponent ahead, especially if it's an aggro deck. Even though some minions like Flame Imp and Voidwalkerhave an advantage early game (especially versus aggro), try to get out the Nerubian Egg as soon as possible, as there are enough boosters in the deck to get that early Nerubianout.

Always count how much HP and Attack a Void Terror will have as you can expect it being a high priority threat in the next turn. If you are playing against a warrior that is holding a Death's Bite, make sure that you grab something else with it than the egg only as it will stay out of range for the warrior to kill. When you are playing against a priest, try to connect it with one imp and the egg so that it has 4 damage and out of the range for any Shadow Word. If you have a Sylvanas WindrunnerCard Name on the board, you can eat her with the Void Terror to get something sweet from the other opponent. Try to clear his board from any useless minions beforehand of course. 

In the Midgame the Imp Gang Boss, Voidcaller and Sylvanas Windrunner are probably the most important cards, and should be played on turn 3, 4, 5 and 6 respectively. 

By the time late game comes around the opponent should be on low hp and close to conceding, as your late game isn't extremely strong. At the end it's the point to "keep the enemy at bay" and to, if possible use a surprise Power Overwhelming + Doomguard combo. Make sure you use the Power Overwhelming first or else it will be discarded with the Doomguard's Battlecry. If this combo doesn't come, our phat badasses Mal'Ganis and Dr.Boom should be enough to seal the deal.

I think that the cards like Nerubian Egg,Voidwalker, Doomguard, Power Overwhelming, Abusive Sergeant, Ironbeak Owl, Defender of Argus require no explanation as they are a part of a normal Zoo deck. Dr. Boom requires no further explanation cuz OP. 

I decided to go for two Power Overwhelmings (Ive seen some decks with one) as I think that it can't ever really be a dead card, except in some cases. Just try to mulligan it if you can as it's more or less dead early game. 

Voidcaller - this card is the cherry on the top of this deck which is one of the best tempo swings that I can find in the Warlock's arsenal. That mini-jizz after you get a free Doomguard on your turn 5 is just amazing. Nuff said. Free Flame Imps and Imp Gang Bosses are never a bad thing. It actually works splendid with the Void Terror, as you can eat it and summon an instant Doomguard. Just be careful not to get the Void Terror pulled out after a Voidcaller dies.

Dark Peddler - The newest addition to the deck, a just sick card that can get you incredible buffs and minions the zoo really needed to be even more OP. 

Imp Gang Boss - A lot of people are judging this card as it didn't really live up to the expectations of the Hearthstone community and still has some "bugs" (DAMN YOU DRUID AND YOUR SWIPE), but I see it as an amazing addition to the zoo's arsenal. Very sticky, spawns more demons, synergizes with Knife Juggler. I don't need more to include it in my secret formula. 

And last but not least, my latest crafted cards and welcome guests of this deck, Mal'Ganis and Sylvanas Windrunner.

Mal'Ganis - This card just has so much value in this deck, it goes perfect with Voidcaller, it goes perfect with any kind of demon as it serves as a jacked Stormwind Champion to them, and makes you immune at the same time? I'll take it. Perfect play is after a good Imp-losion. 

Sylvanas Windrunner - The sovereign leader of Undercity and the Undead. Use her with caution. Make sure you clear useless minions from his board before crashing her into something and killing her. Use her with Void Terror at will as the Void Terror will get incredibly buffed, and you will get a sweet minion from his part of the board out of it. You can even use them one after another on turn 9, as long as you will be getting something awesome from it. 

I can't mention this enough, always try to modify the deck you play to the current meta accordingly. If you are facing a lot of aggro decks, putting one Mortal Coil in or a Shadowflame for the Flame Imp!

If you are facing Freeze Mages or Face Hunters, get a Kezan Mystic in. Use it ONLY if you are sure it's an Ice Block. Big Game Hunter for heavy decks like Control Warrior. 

Some card replacements that some people might not have (I will leave out Common Non-adventure cards): 

Voidcaller -> Piloted Shredder
Imp Gang Boss -> Harvest Golem
Dark Peddler -> Dire Wolf Alpha
Dr. Boom -> Dread Infernal/Sea Giant
Mortal Coil -> Void Terror
Sylvanas Windrunner -> Loatheb/Sea Giant
Mal'Ganis -> Mortal Coil

Note: Don't think to expect such good results if you swap out many cards out of this deck. 

You can also get an Enhance-o Mechano instead of the Flame Imp, and you can get a Bane of Doom instead of Sylvanas Windrunner if you really want to get some RNG going! Or if you just want to keep it safe, just get a Sea Giant in.