This is how i built my teemo. I have stopped playing him often since then, but this build still works quite well whenever i feel like trolling a round as teemo.

With this rune build, it focuses on 2 things. first is teemo's squishiness. this is handled by the armor per level and magic resistance per level runes. The idea behind the per level runes is that teemo has a fairly strong early game with his high dps, allowing him to out trade most if not all other characters early game.

The marks and quints in this page focus on the fact that a hybrid teemo does lots of ad and magic damage, without need for flat damage runes, allowing him to use penetration runes since they scale better into late game.

for my masteries, i focus mostly on AD masteries, but it is a hybrid char, so both of the AD and AP masteries will help. the main part of my masteries is my life steal. The point of the 5 points in the magic damage half of the offensive tree is for the 10% magic penetration. The AP masteries aren't that useful, as the only real magic damage that scale with AP is your poison, which, since the ap scaling on the first tick has been buffed is better than it was, still isnt that great.

i start off with a life steal item to increase the amount of time you can stay in lane, to allow you to farm creeps for money and XP as much as possible. this life steal will not help you that much in a fight with a champion at early game, so dont expect it to. if your opponent has a dorans blade, they will win in an all out fight at the beginnning. at the beginning you will be reduced to pecking your enemies to low health, until level 2 when you can start blinding your enemies and punishing them with you poison.

next i get berserkers greaves for both the movement sped and the attack speed. again, see the movement speed section for why i like movement speed so much.

now comes a malady. sompething this item will turn your character from fail dps to omg where did my health go. the 20 magic damage on each attack will help increase your damage drasticaly, while the magic resistance reduction will increase the effectiveness of your poison. the attack speed and AP are both just bonuses to your dps.

the ionic spark is a good item for the ability to hit more than one target at once. this is good for farming,and it helps teemo be able to take on multiple opponents at once. it is also good to count the stacks on your ionic spark so you can time your pecks to proc the ionic spark as well

by this point in the build, you should be able to demolish any red health bar you see in a matter of seconds. however, you will start to notice that your life goes down almost as fast. thus comes the amazing lifesteal. this item will be your biggest jump in ownage, it will allow you to live longer, and still increase our damage. this items blend of everything is an amazing thing. it gives your poison more damage, your attacks more damage, life steal, spell vamp, and health. add on the active that will make you a deadly assasssin of anyone because they wont be able to get away, and you will love this item.

after this point it doesnt matter whether you get a bloodrazor or a frozen mallet next. both help in seperate ways. if you live long enough, and they have lots of tanky chars, go for the bloodrazor first. if they have fast characters, or you die too quickly, grab a frozen mallet.

frozen mallet - boost health, damage, and allows all of your attacks to slow your opponent.

bloodrazor - increases damage more than frozen mallet, especcially to high health enemies. also gives a little armor for survivability

this is the order for if when you go to the shop, you cant build an entire item

vampiric scepter - vampiric scpeter

berserkers greaves - boots -> dagger -> berserkers greaves

malady - dagger -> dagger -> amplifying tome -> malady

ionic spark - recurve bow -> health crystal -> ionic spark

hextech gunblade - amplifying tome -> amplifying tome -> hextech revolver -> pickaxe -> bilgewater cutlass -> hextech gunblade

beyond this point, order is dependant on what you need

so what i do is i level my ult whenever i can, and make sure to always level poison dart if i can. i put one point into the blinding ability at level 2, then i dont touch it again, until my poison and movement are maxed.

this will give you high dps at the start, with high movemenr speed. the one point in blind is used to allow you to attack enemies with impunity, or to run from them while they cant hit you, if it comes to it.

i go for heal and ignite, just out of personal preference.

yes i said heal

no im not a noob

heal is very useful. especially on teemo. heals main usage is the "heal lure" when you sit in lane with low health, wait for your opponent to charge, heal and then eat their soul. heal can also be used to give you a little more time to get back to your turret. this is not nearly as effective a usage of heal though.
if you really dont like heal, i would go for either flash or ghost. i would not reccomend surge, as it will not be that useful late game, because of your already high attack speed.

ignite is essential. ignite is, in my opinion, the best summoner spell, better even than flash. it can be used as a finishing blow, (this is how i use it usually) or to prevent healing. this will probably grab you 2 or three kills early game, as your opponent runs for their life to their turret, only to get there, then die because they are on fire.

movement speed is the main part of my teemo build. the movement speed built into teemo is usefull in all aspects of gameplay. while on the offensive, it can be used to chase down your enemies, or to get from lane to lane quickly to earn a beautiful double gank at level 6 or 7. as a defensive skiill, it can be used to get to a huge wave of creeps quickly to save your turret, or mainly for getting your squishy *** out of the fight while you are still alive.

the recent release of the use of ionic spark in classic mode has replaced the phantom dancer in this build. the teemo built is no longer built for a lot of movement speed.

pros
high movement speed
high survivability (for teemo)
high attack dps
epic ability to chase (up to a 65% slow, using only your items)

cons
your mushrooms will not be that high damage
early game is quite difficult
your blind isnt very long at the start of the game