Not Updated For Current Season
I haven't seen this build used anywhere else, but it wins me a lot of games with teemo.
It ends up changing teemo into a burst champ with the ability to output enough damage until you can burst again.
Start with dorans blade and almost always go for first blood. The amount of damage you do with each poison shot is ridiculous and add in the exhaust and ignite and its an almost guaranteed kill. This is also where the flat HP runes come into play. It turns teemo into a deceptively tanky champ as you should have more HP than almost anyone in your lane, therefore others are less likely to attack you first when it comes to the initial encounter. While they chase after the (insert lane partners champ here) you will be massacring the squishiest champ on their side with exhaust + ignite + poison + relatively high attack damage from dorans blade. Be sure to save exhaust and ignite for when they inevitably start to run away, keeping the amount of time you can shoot at them to the max.
After you get first blood, because in all probability you will, start harassing normally with poison dart + blinding dart. One strategy i like to employ is sitting in the edge of the bush, wait until the come close enough, pop out for a combo, then dart back in so they cant counterattack.
Next item is boots of lucidity. I would choose these boots because of the CDR. With your runes, masteries, and full items you come to almost if not full CDR. This allows you to spam mushrooms (aka super map control) even if for some reason you don't end up doing well. Berserkers greaves also works if you want to put in a little more punch over time with your poison, but i find the CDR to be a better choice.
Next comes sheen into Lich Bane. This is possibly the most important item to this build, and makes this type of teemo special. Buying this one item almost instantaneously turns Teemo into a burst heavy champ. It relies on using your blinding dart followed by an auto-attack. This will do good levels of damage early on and obscene amounts later, as much as 1k damage with two attacks.
Grab Deathcap next, as it is the single largest AP giving item in this game. You need this burst of AP instead of say an Archangels staff because you will need the rest of your item slots for necessary items. This is not a pure AP teemo. Stacking AP is not what we are going for. Deathcap will keep your AP levels high enough to be a serious threat until you can finish the build.
Next choose one of two items to build, Rylias Crystal Scepter, or Deathfire Grasp. Build Rylias if you are having trouble staying alive, or you cant seem to catch the runners. Build Deathfire if you need to take out a troublesome tank or are having trouble dealing enough damage. Build the other item you didn't build next. Oh and feel free to sell that dorans blade anytime. You wont need it after you get Lich Bane as you will be parting away from auto-attack Teemo.
The last item is your choice. Be it you are having trouble being focused and need that Guardian Angel, or you feel your burst needs some extra umph and decide to get Void Staff. The choice is yours.
HOW TO PLAY: Late game you should be using your speed to harass enemies with your Blinding dart + poison dart combo during stalls before team fights. When you aren't doing this, and because of the uniqueness of this build you could e doing one of three things. You could be placing TONS of shrooms around the map, as you should be poping those out like nobodies business. You could be assassinating squishes as your combo will do about 1k hp or more at max level. You can also be BDing towers as a 500 dmg hit is no laughing matter. One nice thing i like to do for this is save up three shrooms and head to a tower with no minions next to it. USe your first shroom to create an exit path for you and hit the tower once. You will get aggro but it shouldn't be too much. Next back out of tower range and place a shroom just inside the range. The tower should hit the mushroom and during this time you can hit the tower again for obscene amounts of damage. Then either decide you need to back up, saving the last shroom for gank prevention, or repeat step two again.
Well thats the end of this guide. Use it if you like it, at least be informed of this possible build if you don't. I love it because it mixes the best of CDR shroom happy teemo, AP crazy damage teemo, and still retains the ability to 1v1 most people with the help of summoner abilities. Try it out, make tweaks, feel free to mess around with runes/masteries/ability trees until you have it to somewhere you want it and I hope to see you in the feilds of battle soon.