Pros-

Really strong late game
High base movement speed at end of the build
Devastating charge does major damage
Skills allow him to farm easily
Super good ganks with his devastating charge
Friggin awesome looking champion

Cons-

Pretty squishy early game, espicially to ranged
If stunned, he is screwed
Long cooldowns
Skins aren't as good as his basic skin

This really depends on what you are doing. If you are jungling Hecarim, try to get Spirit of dread second instead of Devastating charge. This allows for more jungle sustain, and with blue buff, you can spam it as much as you want. You still want to get Devastating charge at level three, just because it helps greatly in your ganks. Besides that, the leveling of your skills is up to you. My skill sequence is only recommended by me. You don't need to listen to it, but do listen to my first three skills to get.

Flash
I always use this on Hecarim. Even though ghost helps with Devastating charge, Flash also does. I use this instead of ghost for a few reasons. First, if your stunned or slowed, your screwed, even with ghost. I use flash to easily get around walls, or to just juke around enemies.
Ghost
I see Hecarim with ghost a lot. Using ghost greatly improves your ability to gank with Hecarim, but it can be a drawback when your the one being ganked upon. Like I said before, if your slowed or stunned, you can't just pop a ghost and run away. Well you can, but the the slow/stun usually continues into your ghost.
Ignite
I use ignite Hecarim when I solo top. It helps me easily ensure kills on enemies instead of having to run in and spam a bunch of abilities to kill the almost dead enemies. It also helps when you are stuck in a 1v1 melee situation.
Smite
Always take smite when you are jungling Hecarim. Early game Hecarim is squishy, so jungle Hecarim without smite is suicidal. You use it when you want to easily kill blue and red buff, steal dragon and baron (lol), or to dominate the large golems. If you are a troll you can spam this to kill enemy minions. The only backside to this is that late game when you no longer spend a lot of time jungling, you are unable to finish off the enemy with smite. This means that you would need to still run in and spam your abilities
Exhaust
I usually take exhaust when I go bot lane Hecarim. Taking Exhaust on bot lane Hecarim secures kills on enemies while you spam your abilities alongside your carry. I rarely take exhaust, mainly because I don't like to bot Hecarim. This usually involves me losing farm due to the carry needing it.
Teleport
If you do take this, ONLY take it when your solo top. Even though Hecarim is fast, he isnt fast enough to get to his furthest turret in time to stop the fed enemy Fiora and Nocturne from taking down the turret. This is when you should pop your teleport. Save teleport for times like these or times where you are backdooring. When you backdooring Hecarim, just teleport to the nearest ally champion to the lonely enemy turret. Then just knock it down with your high ad and slightly fast attack speed. Then use your Devastating charge and your base movement speed to run away before any enemy even notices that your there.

When you jungle Hecarim, you are jungling a speedy master of death. Get your rampage at lvl 1 and try to get a leash at blue buff. After getting it, immiedietely go to wolves if your on the purple team, or wraiths if you are on the blue team. Kill those, and try to only use your rampage due to the fact that his devistating charge has a 24 second cooldown at level one. When you kill the wraiths/wolves, cross mid lane and head to what would be the other wraiths/wolves. Kill them and then head over to golems. Try using a few health pots here and there to revive your health, because golems deal damage the first time you go up against them. Get them and then back. You should then get boots of speed and your smite should be up by then. Head over to red buff and get it. After getting red buff, use 1-2 of your remaining health pots (if you have any) and then head over to bot lane for your first gank.

When you lane Hecarim, you are laning a speedy master of death. There is only one difference in building when you go lane Hecarim over jungle Hecarim. This would be having smite over ignite. Try to farm a ton with your rampage, because it costs little mana to spam, it hits all the minions surrounding you, and it has a short cooldown when you use it more than once in a row. When you get your Spirit of dread, use it when your health is low due to a fight. Even though the health regeneration is capped, a use once in a while can easily regerate your health fully without having to back. Just be careful, using this a lot will melt your mana. If your going to use it a lot, get blue buff.

I created this build for a few reasons. One of these reasons is to have Hecarim, who is a great ganker, be also a great initiater too. Most of the items In this build allow for ability power and attack power, but at the same time armor and magic resist. When you initiate, try not to initiate with your Devastating charge, because it still has a long cooldown when maxed. Try to use it when you are chasing down a low life enemy, or when your team is screwed over and you need to get out quickly. The abyssal scepter allows for ability power and for magic resist. This means that you can lifesteal more off of minions with your Spirit of dread and you do more damage with your ultimate.

When you are farming, farm with his Spirit of dread and rampage. Using these two at once can easily get you 4-5 minions at a time. You can use this combo any time in the game to get money. Hecarim can easily farm game-round, and when he has his heart of gold, he can make tons of money.

Well at the moment, this is my first mobafire guide. I tried out this build multiple times on Hecarim, and it has worked quite well. Please give me constructive feedback if possible. I hope you like it!