Sometimes I pick these runes, sometimes I just rush all the Armor and Magic Pen possible, but I mostly prefer AP runes.

Masteries are definitely the best suited for AP Teemo. Also, if you're wondering why I pick Improved recall then there you go : http://youtu.be/Xq-hALDpv_E

PROS:

1. Very good harasser with basic attacks even at level 1 due to his Toxic Shot;
2. Deals LARGE amounts of damage when at full build or sometimes even faster if fed too much or farmed as hell;
3. Funny kills when you're dead with Toxic shot and Mushrooms :D;
4. Free wards every 32 secs and faster;
5. DERP passive :D'
6. Insanely good counter for AD carries with Blinding dart;

CONS:

1. Gets focused in every teamfight no matter whats your score;
2. Mushrooms eat alot of mana untill level 15 or so;
3. Needs a good start unless you will be useless:;
4. Small basic attack range.

I tried this build in +100 times it alaways pwned, and now, when the Season 3 came out Riot just made their biggest mistake: they made OP Teemo more OP. If you still didn't get it, they implemented an item called "Runaan's Hurricane" :D just by reading its description you are propably thinking about it on Teemo huh?I'll try to make a vid when I do this build and show you guys off how OP this build
on Teemo is!

Try to be in every teamfight that happens, but don't be foolish to rush in the middle of the enemy team because you will get focused no matter what happens. But, when you buy Runaan's Hurricane, you will become such a terror to the enemy team because it applies on hit effects like your Toxic shot which is COMPLETELY overpowered for Teemo. Now it is sooo easy to get Quadras and Pentas with Teemo :D it doesn't even require skill anymore just correct positioning.

Farming is easy with Teemo, especially when you get Runaan's Hurricane which is made exactly for Teemo: attack speed+launches 2 projectiles that add on-hit effects like Teemo's Toxic shot. Enjoy launching 3 basic attacks per half a sec or so!