Hey guys, just a quick remake of my Teemo guide. So I initially made this guide when I was summoner level 10 and quite obsessed with Ranged AD Carries. That's how I actually played Teemo at the time and it worked pretty well for me. However, I believe that an AP Teemo is far, far better as your toxic shot (which you should be levelling first) increases the damage of your basic attacks with AP.

Pros:

Good Early-Game
High Damage Output
High Attack Speed
Mushrooms Replace Wards (Unlocked at lvl 6) and are also a great damage output/CC
Move Quick acts as a good Escape/Chase Mechanism

Cons:

Easily focused in team fights.
Squishy!
Extremely susceptible to CC.
Requires a good early game to do well.

Greater Quintessence of Ability Power

In combination with an early Amplifying Tome, these runes can give you enough burst early-game to take your lane opponent to half health at level 2 with a blinding dart-basic attack combo.

Greater Mark of Magic Penetration

Good both early and late game as it counters any early magic resist runes that AD Carrier have from runes, and magic resist items late game.

Greater Seal of Scaling Ability Power

AP per level makes for mor AP late-game.

Greater Glyph of Scaling Ability Power

AP per level makes for mor AP late-game.

Berserker's Greaves

Average movement speed.
Good in combination with your W passive movement speed.
Increased attack speed is your first step toward being a solid counter to AD Carries.

Nashor's Tooth

Attack Speed-Ability Power combination helps you easily counter AD Carries at a low level and low gold cost.

Rabadon's Deathcap

Getting this early game generally means your team is going to win.

Zhonya's Hourglass

Good vs. AD Carries and also a great source of Ability Power.

Abyssal Sceptre

Awesome late game for three reasons - Good AP source, passive combined with Magic Pen Marks shred AD Carries' Magic Resist, Magic Resist makes you more viable against other AP Carries and Casters.

Hextech Gunblade

Final Item. Each basic attack will be dealing 341 Damage with 15% Lifesteal and 20% Spell Vamp.