Before you skip over this guide
Just give it a chance, I have been playing Heimer ever since I started playing this game. I know a lot about the champ and how he currently plays. Give it a try before you jump onto the heimer bot bandwagon and please explain specifically if you disagree with my arguments.
I am not the best at this game, but I can do alright, as seen in this stretch of win streaks all using this build:
Why top? Heimer owns bot!
Heimer is the best defender of the game. Why should he be stuck defending the easiest point on the map? He should be top defending the hardest point on the map to defend.
"But he pushes bot like none other." True, but this leads him to become easily over extended and ganked. Furthermore early game he might be ok on bot, but as the game drags on his opponents one shot his turrets. A heimer without turrets to help him does hardly any damage and gets pushed off his point very easily.
Heimer has a passive healing aura, which benefit his teammates when playing near them. He also can blind, slow, and sometimes stun multiple people. Heimer should ideally be an observer during group fights, placing turrets around the enemies, ulting, blinding and rocketing them from afar. Once the fight is over, it is then easy to pick up multiple stragglers with your rockets.
Item Discussion
I used to use 
Prospector's Ring, 2 
Health Potion and Boots of Speed. Since you get health, mana regen,speed, and ability power. However, I found that buying Meki Pendant with the loss of 30 AP seems worth it. This is because you can build it into 
Fiendish Codex much faster than you can build 
Glacial Shroud as per the old guide. The lose in armor is made up for the gain in 
Catalyst the Protector and 
Rod of Ages. Zhonys's hourglass is also still in the build for armor.
On your first trip back buy Boots of Mobility and start building Rabadon's Deathcap. This will give you incredible map control that only boots of mobility can afford you. Rabadon's Deathcap will make it so you are dealing out hefty amounts of damage as soon as possible.
Now I used to buy Sorcerer's Shoes, and the extra damage you get from them are really awesome, however lately I have been having a whole lot of success with Boots of Mobility. Boots of mobility gets you anywhere on the map that you need to go and lets you pick up stragglers like none other. The more I use them the more I like them.
Standard if they have lots of CC and are worried about it.
Once your deathcap is done, you will really want Cool down reduction. I wish I could get CDR sooner, but it's just not worth it without doing more damage per attack. Buy Fiendish Codex. Next it depends how you are doing and what you are worried about facing. If you need Magic resist, buy a 
Negatron Cloak, if you need more armor, buy a 
Chain Vest, or if you just need some more heath buy a Catalyst the Protector. However, if you are doing well buy a 
Needlessly Large Rod and build it into Zhonya's Hourglass. The active on zhonya's will make it much harder for the other team to focus you.
The rest of the items are up to you, I usually finish off my Ruby Crystal by building it into Rod of Ages. Then you can build Abyssal Scepter if you want some more MR and want to deal a little more damage. You can finish your Fiendish Codex into Deathfire Grasp to deal with tankier opponents. Rarely will you ever have the chance to even buy Deathfire Grasp, but when you do keep in mind that you will be wasting about 5% of the CRD. However this is negligible since you will now be dealing 49% of the target's current health in magic damage with Deathfire Grasps active when you have your approximate 640 ap.
I don't buy Rylai's first anymore because the amount of damage that a deathcap provides is just too juicy to overlook. Besides, the slow only procs on rockets and grenades. Plus since we are already a fast heimer, the slow isn't as crucial. That being said, it's still a great item, that I will buy late game on occasion for the health and AP.
If you notice your opponents getting wise and building MR this is always a good choice. However, usually people disregard heimer as a threat and go Armor instead of MR.
Skill Sequences
At the beginning of the game put a point into each skill so you can deal more damage at top. Then get to lvl 3 turrets. Next max rockets for loads of damage. After that max grenades, and finally turrets. Taking a point in your ult whenever you can.
I used to max turrets first on Dominion because they are much better in this game mode than in rift. However, they are still not nearly as powerful as maxing rockets first. I then max grenades since the turrets just die in two hits anyway. Grenades function as an important source of burst damage/blind/stun. The more points into it you put the longer the blind is, the easier it is to get out of ganks, help your teammates, and stun/kill the enemy in front of your two ulted turrets.
The key to a good heimer is being able to predict the movements of your opponent and landing your 
CH-1 Concussion Grenade more often than not. Remember to use this skill when people are meleeing you or chasing you. Another key time to use this spell is when your opponents are all clustered up in a group fight. Being able to blind 2 or 3 of them and possibly stunning one can turn the fight to your teams favor.
Summoner Spells
This spell is a must have in Dominion. Not only does it help you to get you where you need to go fast, but also is a great escape tool. Furthermore you can use it to kite people around your turrets, a function that I don't think everyone realizes, or else they would pick this higher than flash.
Such a great spell! I never understand why people don't always pick this spell. It single handily can save a point from being captured when it otherwise would have no chance. Furthermore I can pop it when there is a fight under our tower to deal extra damage to their team. Or just really punish someone who thinks they can finish off a heimer under a tower. One stun/blind, turret, ult, turret, rocket, and garrison later, they realize their mistake.
Of course summoner spells are mostly up to preference, here are some others that are good that others may prefer.
What do you do top?
Even with this build heimer still doesn't deal much damage so it's important that you understand your role at top. You are the staunch defender of top. You help push your lane with your teammates by clearing minion waves fast and keeping them pushed with you turrets.
You are the main capper of all towers. When you get to a tower, place a turret and start capping. Your turret will aid in the ensuing team battle, and help protect you. Furthermore it allows your allies to tower-dive without aggro from the tower or loosing an important damage out-putter. This works out great for heimer since he gets most of his DPS from his turrets anyway and can still cap. Every time you get disrupted ult/grenade/rocket/refresh your turrets, not always in that order.
Turret Placement
If you are taking top and want to block off the creep, place your turrets where the Blue dots are. Throw some grenades and rockets in their face so your turrets last longer. This will keep their creeps busy while yours come up and help you cap.
If you were pushing with teammates here is a pretty good placement which should help protect you while keeping their creeps busy.
How to be Convincing
If you are planning to go top, tell your teammates right away once you soft select Heimer. I always just say "Does anyone want bot? I prefer top." This lets them know that it is ok to pick their bot champion.
Sometimes there are people who prefer bot and you'll be free to go top. Sometimes they just leave thinking you are trolling. Other times you can't get anyone to go bot, so you're stuck. If this happens don't worry, the only thing that you should do is buy Sorcerer's Shoes instead of Boots of Mobility and you should still do fine. Going top is just a preference since I believe heimer to be stronger their.
Build Guide
Discussion (4)
Athene's Unholy Grail will replace 
Frozen Heart. The ~5% in CDR you lose when finishing the build is negligible considering you wont be making this item too often to begin with.
Swiftness
along with runes and boots help your champ move just as fast as most other champs. I don't have any points in meditation because you should always abuse the health shrines for mana when you need it. Plus the points in swiftness will make you just a hair faster which is always useful.
Summoner's Resolve
. So I yanked a point from
Meditation
in order to have a more punishing 
