Hey guys, I'm CalMeAtlas and while I may not be a well known summoner (yet, hopefully), I do love the champ Zac and I thought I could share here what I've learned playing Zac in just about every position there is. While Zac is primarily a jungle champ, and that is what I will focus on in the guide, he can also be a strong top or support given you know what you're doing.

Pros:

Great Team Fight Initiator
Dirty Ganks
Great Damage for a Tank
Best Tank (IMO) in Game
Ridiculous Passive
Don't have to worry about mana
Sooooo Much Fun
Make your enemies wet themselves every time you think of ganking

Cons:

Elastic Slingshot(E) can be difficult to land
Have to be map aware
Difficult to come back if you get behind
Passive Cooldown of 300
Doesn't benefit as much as other champs from items with mana benefits
You literally have no spine

Smite: As a jungler you're pretty much going to have to take smite. It makes jungling far easier for you early game, allows you to pick up dragon or baron earlier than you would be able to otherwise, and also gives the opportunity to pull off baron and/or dragon steals which can help turn the game in your favor if it isn't already going well for you.

Flash: The reason I take this as my second summoner spell on Zac jungle is that it gives him the unique opportunity to do some pretty risky, even borderline stupid things, and still get out. Almost never used offensively, I'll use it after using my E into the dragon or baron pit and attempting a steal in order to get back out of the pit right away. If done perfectly you could pull off a baron steal all by yourself, take close to no damage, and get away without them being able to chase you down at all.

Spawn:

I always start with a Hunters Machete, 4 Health Potions, and a Warding Totem.

Hunters Machete:

You're a jungler so you'll be wanting a jungle item, which makes sense, and all jungle items have a Hunters machete built into it somewhere, so it makes sense to pick this up to start considering how little gold you have.

Health Potions:

The four Health Potions are to help with your initial jungle sustain, with Zac you probably won't need more potions than these after backing the first time, but always be open to the option should you need it.

Warding Totem:

This is to help give you sight while securing your second buff, whether it be red or blue depending on your path (which I'll talk about more below).

First Back:

You should prioritize buying Quill Coat above anything else but if you still have extra gold pick up those Boots of Speed.

Quill Coat:

This is incredibly important for you to get on your first back, the unique passive is really nice on Zac because it'll essentially guarantee, between that and you're passive, that you leave your jungle at full health every time you go to gank a lane. Having the ward active on Quill Coat also means you can trade in that Warding Totem for a Sweeping Lens.

Sweeping Lens:

The main reason to pick up sweeping lens is to make sure your gank is worth it. If you're on your way to gank a lane, and you see that the champ you're going in on is acting out of the ordinary, whether it be retreating way before you should be visible, or running right at you, it means they probably have where you are approaching from warded. This leaves you with two options; if possible, continue the gank and score a kill or assist otherwise, use that handy dandy sweeping lens to remove that ward and come back to re-gank in a few minutes.

Boots of Boring:

If you still have some money sitting around after buying Quill Coat then go on ahead and pick up Boots of Speed if possible. Movement speed is very important in jungle but I'll touch on that deeper in the guide.

Normal Full Build:

All of these items are good on Zac, but the order you get them in is extremely dependent on your enemy team. Try to pay attention to not only the kills and deaths happening in other lanes, but watch and see which enemy champions are farming best. The reason to look at this is that when a team fight breaks out you want to be able to soak up as much of the enemy damage as possible for your team, and if the enemy ADC is fed and putting out all kinds of Attack Damage, but you have a full Magic Resist build leading up to that encounter, you're going to have a bad time.

Spirit of the Ancient Golem:

In almost every case this will be the first item you finish, and it has a lot going on so we'll break it down one piece at a time. The reason to take this item above any other jungle item on Zac is that his kit in it's entirety is built to tank and disrupt the enemy team in any way he can. This item gives not only Health(which is great by itself), but it also gives Armor, Cool-Down Reduction, Jungle sustain via the Sapping Barbs UNIQUE passive, additional gold with the Conservation UNIQUE passive, AND it also has a nifty little +25% bonus health passive on it, which is what really makes this item awesome on Zac, at least in my opinion. All of your build (if you're building off of my recommendations here) will have health in every item. The +25% makes those items just that much stronger.

Spirit Visage:

This is your normal Magic Resist based item. It also has a lot going on, so I'll break it up just like I did for Spirit of the Ancient Golem above. A solid +400 Health (which is +500 with Spirit of the Ancient Golem) is always good on Zac, 55 Magic Resist helps you outlast those Ability Power bursts that are so common in this current meta, 20 health regeneration per five seconds keeps your health bar even nearer full, and the +10% Cool-Down Reduction will enable you to spam your abilities and clear jungle even faster than before. The interesting part for this item is it's UNIQUE passive, which increases Self-Healing, Health Regeneration, Lifesteal, and Spell Vamp effects by 20%. This is good for Zac for multiple reasons. In terms of Self-Healing effects, well those are your passive bloblets that pop off everytime you use an ability. Now, they recover more health than was spent no matter what, so if you're just trying to recover some health just spamming abilities and collecting bloblets will always give you significantly more health when you're done than when you started. Pretty cool stuff. (Note: Even before getting Spirit Visage your bloblets will allow you to recover more health than you spent using abilities, however, now there is a discernible difference between health spent and health recovered when near full health.)

Sunfire Cape:

I'll typically grab this item for my Armor because it's relatively cheap, gives health(always a plus on Zac), and the passive is really fun for any fight you're in. +45 Armor isn't great, but it isn't terrible, especially for the price and the cool UNIQUE passive which allows you to contribute at least a little bit of damage to those mid-late game teamfights. This can be switched out for other armor items which I'll explain deeper in the Situational Items section.

Warmog's Armor:

This item is FANTASTIC on Zac. +1000 Health (+1250 if you have Spirit of the Ancient Golem), and the UNIQUE passive of restoring 1% (1.25% with So***) of your max health every 5 seconds. Zac is a health based champ, if you didn't know that already I'll continue to reiterate it as I go, and having this much extra health is just awesome. Not only that, but also a very large amount of health recovery. Fantastic is all I can say. I do not recommend replacing this item with anything else, ever.

Rylai's Crystal Scepter:

This item... oh this item. Your "last item" or whichever item you decide best fits you last could replace this. I actually prefer Rylai's Crystal Scepter simply because it's more Health and also for the slow off of the UNIQUE passive. Your primary role as a Zac in the jungle is to tank, and while you're tanking to try and apply as much Crowd Control as you can. This item ensures that no matter what you're doing, you'll be at least slowing the enemy champions around you. There is a nice 80 Ability Power that this item also gives you, however, Zac has pretty poor scaling in terms of damage in general so that won't make a huge impact on your damage overall. I've replaced this with many different things depending on how the game is going with just as much success as this item. I'll talk more about different Situational Items below in the Situatinal Items section.

Oracle's Lens

This item is interesting. I will typically finish this when i'm forced to back to base and don't have enough gold toward my next big item but do have enough for this one. The reason behind upgrading it is the upgraded version lasts longer, has a larger Area of Effect, and half the Cool-Down the free version has,l so it is definitely worth upgrading at some point in the game.

Jungle:

For Jungling, I'll generally take 3 Quint's of Ability Power, 9 Seal's of Armor, 9 Marks of Magic Penetration, and 9 Glyph's of scaling MR. These give you a nice little AP boost to start the game, as well as some armor to take on those neutral monster camps as a jungle. The scaling MR is good for when you start ganking, you won't have to (even though you will) buy magic resist items right away and even gives you the opportunity to gank multiple lanes before having to back.

These are also not permanent, now with the 4.12 patch changes I've been trying a few other combinations but these are what I run when I'm really needing something reliable.

I'll find a way to put a picture of it in here so you can look at them while you read but I'll explain the general idea of what I did here in the meantime.

Jungle:

Zac jungle is generally used as a super tank with great utility abilities that still do damage even if you are at 6000 HP. This Mastery tree is 9-21-0 and that is what I will almost always run for Zac, there isn't really any reason to do 21-9-0, although you could, he is just such a great tank that you shouldn't.

Defensive Tree:(21 Points)

Recovery(2 Points)- Zac is a Health based tank, therefore he will most definitely need some HP5 to keep him going.
Enchanted Armor(2 Points)- If you are playing Zac as a tank, which you are if you're reading/using this build, these two points in Enchanted Armor will give your tanky items some additional... umph.
Tough Skin(2 Points)- If you are jungling and you are tanking (which you are) then taking less damage from neutral monsters is important for getting through those low level neutral monster camps. Also, this opens up the opportunity for putting a point in "Bladed Armor" which you'll take.
Veteran Scars(3 Points)- Zac is a health based champ so giving him some more definitely couldn't hurt, plus it allows you to take "Juggernaut" (which you will) which gives you even more health.
Bladed Armor(1 Point)- I mentioned this earlier, but it's importance for jungle is pretty high. It'll make your clear faster making you have to spend less time in jungle, always a bonus.
Oppression(1 Point)- Your Q slows enemies and you'll use that in just about every altercation you have with the enemy team so taking a little off of the damage coming in always helps as a tank.
Juggernaut(1 Point)- More health = More Tank = More Zac = More fun.
Hardiness(3 Points)- A little extra armor helps with early game survival in those jungle camps, and spending the points here is better than some of the other things offered later and helps you get to that last point.
Perseverance(3 Points)- HP% is, IMO, Jungle Zac's bread and butter. You could gank a lane with full health, take a TON of damage, then recover close to all of it transitioning to another lane. Very strong and I always recommend taking something along those lines.
Reinforced Armor(1 Point)- You need one more point in this tree in order to have enough to get that big last one. This one is up to you, I personally took this one because ganking bot lane and making their ADC hate its life is not only one of the greatest feelings ever, but can be extremely helpful to your team.
Second Wind(1 Point)- Zac's passive, and with how his abilities spend his health, excels at tanking even when he has close to no health left. This just furthers his awesomeness but giving him some additional health back when he gets low. Always fun.
Legendary Guardian(1 Point)- The 21st point in defense is important not only because your team needs a tank, but also because your team needs a tank that can stay in the battle. The additional reduction of CC by 15% lets you really stay in that team fight and helps you make use of your great ult, as well as your kit in general.

Hey guys, sorry this is still a work in progress but I promise it'll be worth it when I finish. If you have any constructive criticism, maybe something I missed or misspelled (Come at me Grammar Nazi's) feel free to let me know. This guide is intended to help new players to both the game or just Zac in general and constructive criticism is always appreciated.