An Explanation for Heimer's Build
Alright, so basically a lot of thought went into this guide and these items are the most beneficial to Heimerdinger's gameplay in a STANDARD build. You should always use your last two item slots for counters against the enemy teams most fed players.
So, let's get to explaining the items.
Rod of Ages: With the Catalyst the Protector being your first built item, this gives Heimer a great advantage in any lane, making him more stable and able to take a lot more damage before he's forced to retreat. Along with that, Heimer's leveling is very fast-paced, as his turrets do most of the work for him. The passive from Catalyst the Protector makes this a breeze for him, and sometimes negates all need for Clarity as a summoner's spell.
Rylai's Crystal Scepter: Rylai's Crystal Scepter is again another item that makes Heimerdinger a lot more sustainable. Not only does it provide him with a bit more edge over enemy players, but it also gives him the ability to slow down his enemies and land his grenades more accurately. It's a must have item for Heimerdinger if you're the offensive kind of player.
Rabadon's Deathcap: Explains itself. Rabadon's provides Heimerdinger with most of his ability power and makes his Hextech Micro-Rockets very good for keeping enemy players at bay.
Morello's Evil Tome: By this time of the game, some cooldown reduction greatly benefits Heimerdinger in getting his Hextech Micro-Rockets off and his grenades for that extra damage.
Abyssal Scepter: Personally, I have never liked the Abyssal Scepter. It doesn't do much for me, but it is an awesome counter-item if you want more of that Ability Power on your side, plus some defence from the dominant mages on the other team.
NOTE: Your last 2 item slots should always be used to build items to counter the most fed players on the enemy team. This can sometimes alter the outcome of the match and can also give you a great advantage.
E.g. AD Heavy Team = Thornmail + Randuin's Omen
AP Heavy Team = Force of Nature + Banshee's Veil
and so on so on. Heed my warning!
The choice of Runes I have used for Heimerdinger are pretty self-explanatory, but I'll explain them to make sure everyone knows why.
Magic Penetration marks come in very handy for dealing that extra damage against squishies like Twitch, and the mages that carry nearly no Magic Resistance. Also, with Sorcerer's Shoes and the 10% from your masteries, it provides an even greater advantage. If you want to really wreak havoc early game, try taking a Haunting Guise as your first item instead.
Mana Regen p/5 seals are a must. Even with Clarity, we want to save this summoner spell for a really important move, so having that extra mana regeneration on Heimerdinger is truly helpful. Note: Heimerdinger FEEDS on Mana.
Ability Power glyphs are used here to give Heimer that upper advantage early game. Because we take the Sapphire Crystal, we lose the advantage of a Doran's Ring and it's AP boost, so the ability power glyphs and Quintessences give him that little bit of early game power.
Masteries are simple.
In the offensive tree, we build solid Ability power, taking straight Ability Power, % Magic Penetration, Ability Power per level and increased damage %.
In the Utility tree, we take the extra mana per level, along with the tome for Clarity to increase the recovered amount, followed by 2% movement speed (Great if you're going mid to avoid ganks. If you aren't playing mid Heimerdinger, I suggest taking the +3 mana regeneration and .5% movement speed) and the increased time on Jungle buffs.
Basically, we want to max Heimerdinger's Turrets first. This makes it easier for Heimerdinger to farm, and gives him the upper hand in acquiring items faster than most players. The reason for this is that the Turrets deal splash damage at level 5, and with that bit of starting Ability Power, it does wonders for Heimerdinger's final creep score.
Rocket's are the next skill to be maxed, and make Heimerdinger a great magic harass. Heimerdinger can easily keep enemies at their towers with his Rockets, and they are the best harass skill he has.
Grenades are great for team fights, and are even better when Heimerdinger has his ultimate activated, making them move faster. We usually max this last, as the Rockets and Turrets are more beneficial to Heimerdinger in terms of Minion kills and Harass, and Grenade's really only become useful later on.
As usual, his UPGRADE ultimate should be leveled as soon as possible, at levels 6, 11 and 16.
Clarity is usually only used Early game and sometimes is not even beneficial to his gameplay. If you know how to play a mage properly, I suggest taking another summoner skill like Surge, or Flash.
Teleport is an essential Summoner Spell to Heimerdinger using this play style.
Heimerdinger is a great pusher, and taking Teleport allows you to get up to towers quicker, and take them out before the enemy even makes it up to you.