In my opinion this is the best Guide for Teemo, because he's not as squishy as ap-Teemo and not depending this much on the enemies armor, because he does a lot of AP and a lot of AD damage.

If the team is a hard AD-team don't take the Mercury's Treads --> take Ninja Tabi

After this u shall buy Phage (Frozen Mallet) because u habe a slow on your basic attacks und you harass ur enemy like hell. With Nashors Tooth you have the attack speed and the ability power that you need with him.

With Guinsoo's Rageblade you have a big AD and AP damage so it's useful for the midgame.

Because you are doing this high damage you are important for ur team, the Guardian Angel makes you more tanky and gives you the revive that will win you the teamfight.

The last item (Madred's Bloodrazor) can be changed like u want:

If the team is tanky take Madred's Bloodrazor.

If u wanna do more AP damage take Rabadon's Deathcape.
(useful if the enemies don't have much magic resistance)

If u wanna do more AD damage take Infinity Edge.
(useful if the enemies don't have much armor.