So, as everyone knows, Teemo is a fast, but weak, support, with his trademark mushrooms and poison. I have included 2 different, and yet not so different builds here. The other fitting well for mushrooming and blinding shot using, and the other for ranged AD, with still some AP to boost Teemo's poisons and mushrooms effect.
This build also a tribute to Mejaj's soulstealer. A great, cheap item that give a lot of AP and even CDR.
Teemo is a very quick champ, making him great escaper, which helps stacking Mejaj's and letting him begin his building with a variety of items, such as vampiric scepter, regrowth pendant, rejuvenation bead, doran's blade etc.
Teemo is also able to be AD, AP or hybrid champ, giving you a chance to adjust to your team.

Please comment.

Pros:
+Both have some survivability (Warmog's effects, and BT's lifesteal and hx.tch's spell vamp)
+Both have nice AP
+Both have ok movement speed
+APish is great for chasing (Having hextech's active effect, and mov.sp.)

Cons:
-As it is Teemo, he will be weak
-Not so great damage (But will surely kill)
-Carryish (begining is begining...)

The runes that have been picked for this build are not so great, maybe... The penetration runes could be ASp runes or crit damage or crit chance runes. Quints should change along with marks. Defence runes are pretty useful though, so they are pretty good.

Masteries here give some ASp and damage, but also boost AP. They also give some much needed def and res.

Ignite is a pretty basic killing spell for attackers, so maybe i don't even need to explain why it has been chosen. Although note how well ignite works with Teemo's poison (^.^)

Teleport is kinda optional spell for this kinda Teemo. It is useful, when you have mushrooms all over the map. Teleporting to those mushrooms gives so much utility. It helps ganking, finishing targets and moving from place A to B ( or even to place C).

In the begining, Teemo can hide with his passive in the middle of a lane, waiting to make an ambush. This is usually pretty effective. This can only be countered with AoE skills (and so only if they know where you are) or with wards.
Teemo is weak. Keep that in mind. And he is also almost always targeted 1st, because of his damage and his blinding darts. This means that you should stay back and place mushes, shoot whenever you can and use your blinding darts on your opponent teams AD champs.
Teemo will also most likely be the one chasing the weak escaping champs, because of his great speed.