Well hi! Teemo's been a favorite of mine and a friend of mine for a very, very, very long time. This is mostly due to his fun gameplay, awesome creep scores, and the ability to annoy the hell out of your enemies when played properly. Teemo's an awesome counter to those AD carries that rely on auto attacks to deal most of their damage as well as burst carries that proc on-hit effects such as talon or wukong. Most of this should be explained later. Any questions should be left in the comments below and criticism is accepted with open arms!
Please do read carefully as detail is something I make sure of just so that the audience can be satisfied.

"YO DEM RUNES LOOK LIKE AD CARRY RUNES", which they are. I enjoy them on teemo instead of the normal mage rune build because teemo is more of a damage-over-time and an on-hit champion. Thus, armor penetration while maxing Toxic Shot should give you a great amount of damage in the laning phase and team fights. Armor Seals and MR per level Glyphs can be self explanatory as Teemo is one of the darn squishiest champions in League of Legends. He is not so mana reliant as long as you do not spam your spells and get blue buff once in a while. Thus, flat armor and MR per level will give him a sustain that favors in team fights and tough chases.
Remember how I said Teemo is more of an on-hit or DoT type of champ? Well why not get those attack speed Quints to get you that early domination you probably want in both harass and farming!

This does not require so much detail, Toxic Shot will either make or break first blood if the opportunity comes to your lane, as it will deal the most damage and can scare away enemies even if you're close to dying. Max the Toxic Shot first, Blinding Dart second, and Move Quick third besides your ultimate move. Remember it is pretty vital to take a point in all three early. He needs the speed to chase or run away from enemies as easily as possible ESPECIALLY early game.

A short video guide to using Noxious Traps will be created soon and posted.

Begin: Dagger + Health Potion Health Potion Health Potion

A Dagger first reveals his squishy 4 health bars early, but allows him to easily get early farm and easy damage on your lane opponents when possible.
Go for Beserker's Greaves obviously, and work to a Nashor's Tooth. This will provide a plentiful amount of Ability Power and Attack Speed for him early game. Next, go for the Recurve Bow and Phage for the possible slows, health, and moar attack speed :D Build to the Frozen Mallet for the garenteed slows and health gains as well as the Bloodrazers. Then, top it off with a Rabadon's Deathcap and Lich Bane. As Teemo can have low cooldowns on the Blinding Dart as well as his mushrooms when he stacks three together, activating the Lich Bane should not be a problem. Remember to not get Lich til the end because to gain the max damage on its passive on-hit active, you need to stack some Ability Power beforehand.

Now you may be wondering,"This looks like an AP teemo with a lot of attack speed on the side, so why the random Madred's Bloodrazers in the middle?"
Teemo's weakness Meta to Late game is the tanky bruiser, such as Cho' Gath, Olaf, Talon (if he gets some health), Renekton, and even normal bruisers like Nidalee. With them slicing at Teemo, he cannot do enough damage to help out the team much, which might lose them the fight. Stay away form the fight for a bit until most abilities are spent by the enemy team and unleash your attacks as much as possible on the carries and then the tanky bruisers. This Madred's Bloodrazers combined with your Toxic Shot will create some very nice damage on your attacks against these bruisers as they are focusing other people. With them out of the picture, you are not facing much of a threat in the fight any longer. Keep that in mind :D

He has a Ghost so take Flash :3
Ignite + Poison = Dayum

*I may be exaggerating a tad with the ignite, but it'll make or break a fight with the DoT*

Pros:
-Swift Scout
-awesome harass with Blinding Dart
-can blind an ad carry while fighting, rendering them useless for a few seconds
-His mushrooms are so great for both offensive and defensive playing
-Farming ability is godlike
Cons:
-So ******* squishy early game (a little less late game)
-An oracles will destroy your shrooms :c

In team fights, please remember one thing: Come in a bit later than your initiators/fighters. You can actually come in closer, stealth with your passive, and dart back out and deal the damage you want. Just make sure to blind their AD carry ad focus down the squishies without getting to close to the fight.

Another great thing about Teemo is the kiting. His blinding dart has a long enough range to use without losing too much distance and his mushrooms are just self-explanatory with the nice damage over time and the insane slow.

His passive could save you in a fight. If you can dart quick enough ahead while being chased or waiting for a low health enemy that just escaped a fight, you can stealth and make them feel safe. You randomly pop out and be like "Well hi there!" and demolish what's left of their health bar. Although this may seem like the cheap way to kills, it's still a nice kill for you and it is NOT the only way to get kills with teemo.

Honestly, I think I've explained quite enough about my teemo. In general, if you could not tell, I go for an AP/AS Teemo, abusing his attack speed and AP ratios on his abilities and mushrooms while taking that frozen mallet for his sustain in large fights. I hope you have fun on the fields of justice, and remember, "Size isn't everything"!