This build maximizes both ability power and attack speed, and enables Teemo users to dish out steady streams of magic damage. I've used the latest vetted Teemo builds which seem to either consolidate damage for mushrooms [AP] or auto-attacking [AD]. So my question simply is: why not have the best of both in that aspect?

Teemo can auto-attack for bonus magic damage as well as time-based damage from his passive. He can also have very strong AP for his mushrooms, which makes Teemo the best possible AP variant auto-attacker without losing any of the mushroom damage because the user chose to go AD instead. Well this build shows Teemo users the best of both worlds.

Magic penetration has proven to be the best choice simply because the damage output is prime. Therefore, MgP runes are a must-have for him!

Force runes add to Teemo's late game, and combined with both Malady and the Void Staff, Teemo is a force to be reckoned with!

I've had several matches where I forced the opposition to invest in oracle elixirs over and over because this build made my Teemo way too much for them, and as soon as they lost the effect of the elixir from dying, they were 400g further away from their builds.

You can experiment with your own Glyphs and Seals, but I recommend not deviating away from the magic penetration runes as they are essential to making Teemo's damage output effective.

I went with the standard caster mastery setup, but since I use force runes which scale per champion level, I make it mandatory for myself to always make sure and put the maximum points in "Awareness (Utility)" to get to 18 very quickly. If you have a level advantage versus other champions, you also have the extra AP thanks to your Awareness + Force runes.

As far as mana reganeration goes, the Doran's Ring plus Nashor's Tooth should be enough to keep you out there for quite a while. But I strongly recommend you get blue buffs whenever possible to aid you in mana, as well as pump out more mushrooms. More mushrooms is never a bad thing! <3

I usually start off with Doran's Ring because it offers the perfect blend of AP, health, and mana regen to keep you out there for as long as possible during early game.

Note: I usually rush Madreds Bloodrazor versus tanky teams. I go with the cloth armor and hp potions to start, and then I buy the claw and normal boots. Next I get the recurve bow for better last-hitting and minion-farming capabilities. Lastly, I get the final pieces for madreds, and then go for the Void Staff just in case they try rushing Force of Nature armor on me to counter. If this is the case, you'll definitely need the blue buff to support the lack of Nashor's Tooth and Doran's Ring because otherwise, you'll have no choice but to play very conservative until you do get Nashors.

After Doran's Ring, I'll get Malady so that I deal extra damage from auto-attacks as well as make my mushrooms stronger. You "can" go for Nashor's Tooth instead, but since it's more expensive, it'll take just a little bit longer to get. That, and the magic resist redux isn't there. It does enable you to pump out more mushrooms, but I just go for Malady because it helps me farm better, making my farming go faster and thus speeding up my item building.

I use Mercury Treads specifically for the anti-cc. But it's also good versus those pesky AP casters, as well as other Teemos [laughs]!

I usually go for the Void Staff next because any good player will attempt to counter you with magic resist (i.e.: Mercury Treads, Banshee's Veil, Force of Nature, etc.). So the best way to fight that is to ignore as much of that MR as possible to maximize your damage output. You can elect to rush the Deathcap or other AP items if you like, but if you want to circumvent magic resist early on, the Void Staff is a perfect pick.

Finally, I go for the Deathcap and Hourglass simply because they're the two strongest AP items in the game. You can get them in any order, too. I usually rush the deathcap to make my mushrooms ridiculously strong, but if you're being too heavily-focused by enemy champions, then the Hourglass is the item to rush first. REMEMBER to USE the Hourglass's active effect only when you're about to enter a team fight. Activate it just as soon as you're focused, and not just randomly. If you use it randomly, you'll have wasted it. I usually make sure I have my "Flash" up before going in to kill the squishiest opponents. If I get focused, I use my active from Hourglass, and Flash the hell out if needed!

Max out your Q and E for the blind and damage. Just one point in your W is enough to get you out of trouble in the meantime. Invest in your mushrooms (or R) whenever possible, which are at levels 6/11/16. I recommend you not deviate from this sequence.

Exhaust: This enables you to 1v1 whenever you'r eattempting to back-door turrets because it significantly reduces damage output and is an effective snare. This can also be used as an escape measure. Your investment in utility makes the recharge on it amazing, but do not use it without a compelling reason to. In other words, use it wisely!

Flash: The perfect way to escape a mean gank, and combined with the strongest investment in your utility, you'll have it that much more often. You can also use it offensively, but remember that once you've used it, you won't have it for another while, and you'll then become prone to being ganked by others. In other words (again), use it wisely!

Ghost is another good candidate. Ignite has good offensive capability in that it's effective versus self-healing champions like Warwick or Mundo, but you'll have less defensive capabilities in taking it over exhaust or Ghost. Teleport is also good if you want to back-door just a tad faster, but it leaves you prone to ganks if you're without flash and/or Exhaust/Ghost.

All of the other summoner spells seem sub-par to me. You can use whichever you like, but I don't recommend it.

Play defensively, last-hit as many minions as you can, and only harass during the laning phase if you see an opportunity. But DO NOT overstep your boundaries because you are a squishy, and have little to no damage during the early-game phase [until you level enough and have some of your items, which means mid-to-late game].

Pros:

Strong AP overall for mushrooms.
Auto-attacking effectiveness.
Amazing back-dooring capabilities.

Cons:

He's extremely Squishy!
Magic resist can make killing foes difficult [hold down the Tab button and check to see who's got magic resistance items].
Oracle Elixirs also make sabatoging the jungle difficult [in this case, place your mushrooms in bushes where you KNOW enemy champions WILL cross; also, plant them on minion waves as they come in under your turret, making it difficult for enemy champions to stop].

Back-door as often as possible, and sabotage the areas where you KNOW enemies will cross in an attempt to gank you.

Plant mushrooms a couple lengths from your turret to prevent enemy pushes. I do this if I suspect a push on a lane. I'll put 1-2 mushrooms in the very center, a couple units apart, to buy my team some time to get there. You shouldn't have to if your team's solid, but if you have to carry some of their weight [because they suck], then plant one or two of them in their lane so that they can defend a bit better.