Hi, it is my first guide here, and my first guide in engish, so don`t banish me for it)
I buyed heca when he came out, and i saw, that a lot of hecarims is a litle useless as full tanks, or autoattack damage dealers. So i decided to try cdr build, and it work fantastically! So i keep playing it, and trying to introduce it here.

Pros:
Great chaser.
Great aoe during the teamfights.
Great vamp in teamfights.
Nice escapes.
Half immune to teemo`s blind, we deal most damage from our skills.
Can move over the map fast with his e.
Fuc***ng ghost of centaur
FRENDSHP CHARGE!!!

Cons
There can be some mana problems cus spamming q.
Need sense of his q range.
Needs good position to do his e effectivly.
Some escape problems with ult (need aim correctly).
He isn`t tanky, he is healty damage dealer, so you will need someone fat in team (usualy jungle, but support-tank like ali will be nice too).
Some problems with ranged top.
Vunerable to cc. Expetialy to silence, but later this will be not so big problem as early.

Heca has one of the most powerfull spells - his rampage deals nice damade during teamfights, but it`s cd with 2 stacks is 2 seconds. Nice, but with 40% cdr it becomes a lesser that a second! Around 300 aoe damage per second! Cdr is also good for his other skills - his vampire w and chase\escape e and r

I think i don`t need to explain - defence gives us some defence at first levels and later. Top usualy is the battle of offtanks, so it will help us a lot early.
Why 9 in offence? we need some % arm pen to deal a little better damage in lategame. 3% ms worse.

Answer - it gives ms, that means better escapes\chase AND damage. At early game it can beat your enemy - at middle of duel turn it on, yes, you aren`t chasing ot escaping, but it gives some damage, that can solve your "problem".

It is the most usualy runes for every top player - armor cuz there is usualy physical damage at top, m res per level cuz same reason - we will meet mages later, and then per level runes will be more powerfull. Or, if there is someone like rumble, you can choose blue as m res and yellow as armor per level.
Arm pen quint and runes greatly help us to deal damage to that champs, who don`t have a lot armor, so in early game they are the most effective.

His passive - we will get damage for our movement speed. Just nice.
We focus rampage, cuz it is our main damage dealing and line winning tool - with it we can just harras enemy. when he comes closer for lasthit.
Second we focus w for it`s lifesteal. Yes, versus champions it works like we have lifesteal and spellvamp (it is great for our q in teamfights), also we gain health when our allies damage them. It is realy nice skill, but it`s heal from minions capped - we can`t get a lot heath from them. You can keep it level as q level if you have problems at top.
E we pick just for chase\escape\movement tool and as some control. Try to attack enemy with it skill later as you can - it will deal a bit more damage, but you know, that "a bit" can win the line and the game.
R - Our ult is realy beautifull, but it needs a litle practice. You can use it as initiate tool, as escape - it`s range is nice and we can jump over the walls. Also it is great controll - ult on ahri\cassa\other escaper when their stun\silence is over, and you`ll get a kill. But it have problem - sometimes you can`t jump over the wall when you doing it "fast", whithout aiming, so you will need some precision.

Max cdr is 40%, so i decided this items. But this items also great cus stats, that they give!

Boots - cdr. We build boots early, cus then we got cdr and can chase enemy and harass him with q + a litle damage.
Brutalaiser - Ad+penetr+cdr. Very good early item.
Sheen - Awesome item. Our q cd is low, so we can use this blue blade fully.
Next item - zeal or phage you can choose yourself. Need hp? Buy phage. But usualy i buy zeal cus ms that it gives.
Trinity - gives us ms, as, a litle crit, atk speed, damage, health, some mana... and empowered sheen passive! All what we need!
Ghostblade - The. Best. Item. It gives cdr, good damage and penetration BUT - it`s active part is true awesome. We will get damage from our passive, and with our e nobody will escape from us! (Cassa, SHaco and Ahri, just sit down. We know, that you can escape from everything. We aren`t speaking about you). Also it gives atk speed, so we willn`t be so useless when silenced
Lathern - Why so late? If you want, you can get it early, but usulay i pick it after core ittems. Why? Because we just need vamp and armor from it. Ad is nice. Free ward is just a good addition.
Then we get mres item (look further)
And the last item is soul sroud.
Maybe someine say -"hey dude, wtf, it is support item". BUT
1- this item gives us last 10% cdr, gives a lot hp and gives manareg. Maybe it will be better for suppot, cus cdr is aura? Ok, ask yourself, can support live all the teamfight and chase with you? I think no. If you think that he can, just get sunfire or warmog.

This is the only question i have for this build. These 2 items are great - all gives mres, all of them we need. One gives us more mres, gives a lot atk speed, so we no longer will be fully disabled by silence, and gives 42 magic damage on autoattack. But the other gives good ad, gives shield, and gives nice ad at low hp. I usualy picks Wit`s end, but sometimes Maw is better (Vs Karth or someone else... I realy don`t know)

Tactic at laning is easy - don`t allow enemy to farm. It is easy - keep near your kreeps on low, so when (and if) enemy come to lasthit - Q,Q,Q,Q! Chase a litte to harras him, and return to farm.
If you sure, that enemy will come closer soon - try to have some stacks from kreeps, but do it carefuly! Try to harm with your q only 1 kreep, this skill pushes hard, and then - hello, jungler!
Always keep an eye on your ward. Jungler will be bad surprise at start. If enemy team have someone likes Shaco (hate him) or Lee, buy ward at start - they can gank you at 2.20. They will be lvl 2, when we lvl 1, and we can`t escape from them fast.
Your ult - at laining don`t use it as initiate, if enemy knows where you are. Use is only as chase and towerdive tool (be careful - try to catch him NEAR tower`s range. Tower hurts, realy. We aren`t tanky enough)
The main problem is ranged top - it is awful for heca. He can`t harras them so nice when they lasthit. But - 2 boot help us a lot. But ranged top is rare. The main ***pain is Vladimir. He can just W away and regain health with q. It is hard.
Also a litle problem is that, who have stuns and dashs. We can`t harras them hard - they dash away, and they can harras us under stun (like Rene). So it will be a problem.

Ganks - Hecarim ganks is realy nice - when enemy away, and you have at least half of your hp and you pushed - try to gank mid. Start e - ult out the brush(sometimes you can just attack with e) - pull him in hand of your mid, and qqqqqq, and if you didn`t use our ult - r (Again, near the tower range).

At lategame we are a litle tanky and we are nice iniciator. Just spam q, and all will be ok.

Teamfights
We are the king of teamfight. Try to focus enemy carry and mage - they are usualy squshi enough, but also try to harm all the enemys with q. Bus also you can save you teammates with your e.

Buff need -
At start blue is realy nice - mana to spam q, cdr... but later we don`t need it so much, and in lategame we don`t need it - max cdr already, and no mana problems.
Red - just nice for us, nothing to say.

So, i tryed to write my first guide here. What will be with it - i don`t know. But this build never failed me.