well..i got this build after so many tries, and this is the result. I hope you like it.

Exhaust is a great choice for teemo, when stealthy can "gank" by yourself using it.

Ignite is the best spell for him, can take so much life plus poison, and btw its great to stop healing done like garen.

9x Mark of Insight - Its great to increase his damage given, with blind darts or even with poison.
9x Seal of Fortitude - It makes teemo become stronger on health...without this runes he is so squishy.
9x Glyph of Potency - This will increase a lil ability power..can help to take more damage with his skills.
3x Quintessence of Potency - Increase rapidly your ability power, like another dorans ring.

First at all I get Dorans ring and boots of speed. Increase your ability power getting blasting wand. Finish your boots into Sorcerer's shoes,to increase magic penetration. Get as soon as possible Nedlessly Large Rod. Now you will need attack speed, to farm, hit faster the enemies anyway, get Stinger. Finish your ability power...into a Rabadons Deathcape. Then finish your stinger >> to nashor's tooth and now your mushrooms has low cooldown and you are hitting faster..put shrooms everywhere. Now you will need some life steal cuz the enemies can be feed or even taking a good damage, then get a Hextech Gunblade which slow their movement and increase life steal, attack damage and ability power. Finally your last item which is Guinsoo's Rageblade...this is helpfull...each stack, increase attack speed and ability power.

Actually, i only get 21 defense cuz teemo needs more health and AP in that case..My runes are nothing without masteries, cuz this is really usefull.

When you are Stealthy with Camouflage, make some traps, shrooms,make sure that he is gonna step up on ur shrooms, hit him with blind darts > taking a good damage with this build already. use Ignite, and Exhaust if it is necessary.