This is my third build, my builds are not really ready, i'm am testing them right now. Since Hecarim is a new champion, it's hard to have a really good build right now, so i'm making one for him. Now let's see if it can get any better.

First buy is Brawlers glove, to get Avarice Blade early in game so you can build up faster. After boots or if you have enough money Berserker's boots to have more attack speed and extra attack power because of Hecarim's basic passive skill. After that, Cloak of agility to buy Cloak and Dagger, to gain cool critical chance and attack speed again. Now let's get serious. Infinity edge Serious damage and critical strike chance and damage. Continue with Phantom Dancer get's really good boosts from it, and finish with trinity force if the game has not yet finished.

This is my basic build for melee champions. Boost Critical damage, attack speed, and some life stealing.

Rampage is the most useful skill of hecarim, low cost fast cooldown and massive damage. Next Spirit Of dread because you can kill lots of minions and healh regen at the same time, great skill too. When tou can boost onslaught of shadows do so, great skill to finish or start killing enemy champions, or you can flee with it. Last Devastating charge, great skill but not my favorite, good when you have some advance, can finish a champ as well at giving starting blow. So What you have to do is to boost them in that order; Rampage, Spirit Of Dread, Onslaught of Shadows and Devastating Charge.