I am going to have to disagree with everyone in this thread and say that Heimer is #1, at least of the heroes I have played. Many people would say that playing Heimer is just button mashing, but that is because they are damn noobs!
He is a glass cannon, and as such there is always a lot of maneuvering. He must maneuver to survive and also to get his skills off. Button mashing is not the way to play him. You need to move at angles that will make your missile hit them. You also have to skillshot your nade, which is one of the slowest skill shots in the game with one of the most devastating effects. Leading a character by a second or two with the nade and landing a hit is very satisfing. Throwing it through walls and firing a missile at the same time is also a skill you need to acquire, since the nade gives vision.
As turrets go, in setting up for a battle it is important to place them at the right time, and also to make fall back positions using them. Popping the ult can make kiting very strong. When you are being chased, a good combo is running into a bush and stopping, placing a turret, waiting for the guy to get right next to the turret and then stunning him with the nade. After that, fire your missile and pop your ult so that he is slowed. With that combo, you can kill anyone that isn’t carrying 150 MR.
In conclusion, heimer isn’t easy. There is a lot to think about in battle, and surviving early game can be difficult. I use magic pen and flat mana regen runes, as well as 3 movement quintessences. For masteries, grab just enough for the magic pen offensive, then go for summoner cooldown reduction (Ghost reduction, flash reduction, and overall summoner reduction). Ghost and flash are necessary for surviving with heimer. For upgrading skills, get turret, 1 level nade, and then finish turret first and missile second. You only need 1 level in nade to get the stun.
Farm with turrets, and at level 6 or 7 grab the blue if it is up. When you have blue, you can start harassing.
For item loadout, go sorc boots -> tear of the goddess -> soulstealer -> Rylai’s -> archangel -> zhonyas -> void staff .
So… that’s basically how I play it.
EDIT: I forgot. Start the game with doran’s ring and a health pot.