This is my Teemo build, I choose to focus around on hit items, which brings down opposite team players slowly in early, mid game and simply tearing through them come end game.

I`ve played around with this build many times and I find it to be very efficient throughout a game.

Pros:
Good map control with shrooms
Good escape mechanisme with Move Quick and camouflage
Good assasin early, mid and end game
Blinding Dart for those tricky situations

Cons:
Squishy
Getting accused of kill stealing when enemies run into your shrooms

My rune choices are based around Blinding Dart, Toxic Shot and Noxious Trap They scale with AP and it gives some nice early damage.
Why I dont select any survivabilty runes:
My playstyle with Teemo is playing safe. I sit back and last hit creeps and hit enemy players when the chance occurs. I only press enemy players when they are running low and when I feel certain I get the kill. Poison ticks, ignite usually gets the good deed done.

With my build I focus on getting Toxic Shot maxed out first, as it`s my bread and butter with this build. It doesnt do alot of inital damage, but it will drain enemy HP quick enough.
with 9/18/27/36/45(+14% AP) on hit and DOT for 6/12/18/24/30(+14%AP) each second for 4 second, Toxic Shot goes through enemy health quick enough if they play reckless.
Blinding Dart as second ability to get some burst and the on hit effect is good when engaging early on. It will neglect a 1-2 white hits from your enemy.
Move Quick either for escape or cathing up to the enemy when they run.

For more early burst, level Blinding Dart first.

Ghost - The running speed is just awesome to have as either escape tool or cathing up. With Move Quick you can get out of or into situation quickly. It also allows quicker roaming time around.
Ignite - To finish of enemies when they are running low. Works well with the build and you skill set.

Ignite vs Exhaust - Nice to catch up to enemies trying to flee, or to slow down attack speeds in 1v1 situations.
Ghost vs Flash - I rather prefer ghost over flash. With movement speed and shrooms, I get out from most sitations.
Ghost vs Teleport - Teleport can stop downtime if you are running solo top or mid, or simply if you want to be able to move around the map to support.

Starting item:
Boots
Health Potion x 3

or

Doran's Blade If you want some extra initial damage from auto attacks.

First Trip back:
Sorcerer's Shoes
and
Amplifying Tome
or
Dagger Which will build into Malady on your third trip back.

Next I start with building Madred's Bloodrazor and I choose to start with Madred's Razors. Nice during creep farm. What items I choose next in the build depends if I want some more survivability in form of health or the extra running speed.

When Madred's Bloodrazor is done I start to build some MR for myself and yet another on hit item. Recurve Bow is first buy and I build into Wit's End

With this as my main build I usuall go for Giant's Belt and build into Frozen Mallet for slow and survivability.

Last item in build depends on what you prefer, I go for Chain Vest to Guardian Angel to avoid beeing to squishy.

I`ve found that with this build I can take down whoever quick enough for them not to kill me. It works find against all champions and builds.

Bot lane w/ support:
1-6 play:
I play Teemo defensivly. I try to avoid taking big risk early game, due to being so squishy. Sitting back and getting creep kills, and hitting enemy champions when the chance occurs is my main focus. Teemos burst is not much to brag about, so getting enemy players low before engaging is adviced. There will always be a risk >< gain to take and jumping on enemy players to soon, will only result in failed attempts.

But with patient play you can bring enemy players low enough that you can jump them. If your lane gets pushed back, it gives room for your jungler to gank aswell.

6-13 play:
Your item build should have Malady and the build towards Madred's Bloodrazor should be well underway. At level 6 you start to put out shrooms. Focus putting them along gank highway(river), so that you can get early warning when enemies are moving towards you. My first shroom goes in the brush just above the second tower, second goes into the brush at the end of the river. Third and fourth are placed by dragon and the exit from their blue buff. This gives early detection and a good map cover for bot lane. Next I start to put them in the lower brushes. This works well towards knowing when the enemies are in there, aswell during gank opportunities if the enemy should choose that escape route.

I also start to push the lane more when I have shroom cover. Early detection gives me the chance to fall back if needed and avoid ganks. At this time, if my game is going well, I will roam around the map, harrasing in other lanes and putting out shrooms to help my team mates.

End game play:
By late game you should have placed shrooms around the map, giving your team good map control. During team fights you work as carry, picking out their carry first and using Blinding Dart to lower their damage output. Placing shrooms to cut their escape routes is another good way to assist your team.

Bot lane w/ carry:
1-6 play:
This will starve your gold income as you will be fighting against another carry. First off, dont pick teemo in such a situation. It will not help your team, but if you insist, play your lane aggresivly. Farm your BOT opponents for gold.

Rest of the gameplay goes the same as playing with support down, but gives you more room to roam about the map, past lvl 6.

Mid:
1-6 play:
Guess what, Teemo works well in mid aswell. But it`s even more important that you play safely. Other AP champions have good burst and it`s easy for you to fall victim to them. Play safe, poke enemy when there`s low risk and push them once they run low on HP.

6-13 play:
Shrooms are place along gank highway(river). When placing shrooms, think like the enemy jungler. Place shrooms at the edge of brushes, usually the jungler doesnt come running from the middle of the brush, they tend to come from the edges. Place shrooms at the exit/entry points of the river. You should still play safe, the burst of the AP hasnt decreased during the game time, attack them when there`s low risk and jump on them once you feel safe you'll get the kill.

End game:
Same as written above.

Solo top:
1-6 play:
Same game play goes here, play safe. Usually you can have an edge over the enemy with your DOT from Toxic Shot. Try to have your jungler help steal red buff. This will give you the opportunity to play a bit more aggresive against your opponent at top.

Mid and late game goes as before. Push lane if you have the upper hand, if not sit back get creeps and wait for your jungler to come.


Tips:
1. Using camouflag before engaging enemy players gives you a good AS boost.
2. Getting red buff will increase your damage output, have a nice chat with your jungler about this. At lvl 6 you can get it yourself without to much trouble.
3. Using camouflag as an escape mechanisme works aswell. Use Move Quick or Ghost to get away and then simply hide in a brush can get you out of sticky situations.
4. Blinding Dart only stops autoattacks from hitting. Use it on hard hitting enemies in team fights.

Playing teemo safe is the key to success. Often I find teemo players acting like bruisers and they tend to feed a lot. Your skill set helps with map controll, lessen damage output from enemy players and with your finale build, cause enough damage to put down most enemy players.

Always have shrooms down at key locations, as they can provide map controll, good escape routes and take down enemy players running from fights.

I've had success with this build and with the latest teemo patch even more so. Factors that ****s me over, is when I get cocky and make bad calls in fights. You'll still be squishy and thinking you can molest everyone will get you into trouble.

I know teemo guides are a dime a dozen, still fun to write one ;) Enjoy!