This is my Hecarim build, i build him this way every match and only got positive stats with it.
Precaution: I DO NOT RUSH A TRIFORCE FOR TONS OF DAMAGE, neither i get a triforce anywhere in the game.(In fact i beat Triforce Hecarims dozen of times with this build.)
Precaution2: I DO NOT JUNGLE HECARIM BCS IM COOL

Anyhow enjoy my Hecarim build :3

SUMMONER SPELLS
Ghost and Ignite worked well in the ~ 10 games i played with hecarim.

SKILL SEQUENCE
I max Rampage right away because it's your main damage source.
At level 2 i take Devastating Charge which allows me to harass easily by rushing in with it hitting the enemy with autohits + a few Qs.
At level 4 i take Spirit of Dread and max it second.
Of course you take Onslaught of Shadows whenever you can.

RUNES
Greater Mark of Desolation for your damage.
Greater Seal of Armor Armor yellow(pretty standard for solo top)
Greater Glyph of Magic Resist Mres per level since casters grow stronger in midgame and up.
Greater Quintessence of Attack Damage I personally run with these since they add some great early damage for last hitting and harassing.
U may aswell go with Greater Quintessence of Movement Speed or a mix of the Armor/Mres quints.

ITEMS
Boots through your passive you are granted bonus AD and the three HP pots are pure sweetness.
The Brutalizer heyo renekton, how u doin? tons of dmg in your early game? how? Brutalizer u say? cool.
Mercury's Treads You can either grab these or Ninja Tabi depending on the enemy team. If you are playing vs total Newbies or really really bad players you can also roll Boots of Swiftness but i don't recommend them since the AD gain through ur passive is not very high, and the MS is non noticable.
Philosopher's Stone MP5 and HP5 + GP10 GREAT!
Youmuu's Ghostblade first major purchase works great in a 1 v 1 or a chasing/fleeing situation.
Shurelya's Battlesong second major purchase with massive HP5 and other goodies. But the thing you really want here, are the HP and Movespeed.
Bloodthirster Hue Hue Hue sustains you in battle or helps you to retaliate from one. The Damage is ofc pure sweetness.(2 times!!)
Guardian Angel if neither the AP or AD carries are fed. Last Whisper if 2-3 tanks are fed, Randuin's Omenor Thornmail if AD carries are fed, Force of Nature if the AP carries are fed.

HOW TO TRADE BLOWS
If they come too close for a last hit Rampage them and maybe add some autohits followed by another Q. If you are confident to get a Kill, E -> Q -> AA -> Q -> AA -> Q -> Ignite.
Use ignite only if you are sure you can get the kill, if you need ignite for the kill, meaning that the enemy got heal/a strong selfheal or tons of HP pots, use it before and make sure to kill them then. If your lane enemy is rather good you should be able to grab a kill as soon as you reach level 6. Remember that you can Q when you ult Too.

Hecarim is a great assassin tho 3847239847 people will rage about this and probably downvote, hey idc :D
If your team initiates a fight you blow Ghost/Shurelya's/Youmuu's and your E to gain up to 1k movespeed which results in tons of damage with your passive. Rush to the enemy AD or AP carry(whichever causes more trouble to your team) and make sure to Q them once before you hit them with E since your E grants massive AD with your passive.(dat rhyme) Then hit with E and then spam Q keep W up and use your ult if your prey Flashed/got away with something else, if not just finish the Carry and blow ur ult into the rest of their team to split them and mopp them up.

Ganking Mid from top is actually pretty easy too, i think you know how to gank.

Yeah thats about it, make sure to try the build out since the movespeed assassinations are TONS OF FUN, especially when the enemy ad carry is like: O_O where did u come from?

I Hope you'll have as much fun as im having with Hecarim and i hope you'll have even more fun with this build^^

PEACE OUT

Olaf gives u a hard time... request some early ganks, hes OK to handle then.
Pantheon hard too but easily countered with Armor
Garen EWW this ones Ugly to face.

I did beat: Irelia
Ahri
Hecarim
Olaf with tons of Ganks... still worked out and i Won.

No more experience :D