This guide is was adapted from the statistics at http://lolalytics.com/champion/Teemo/Support/

Note: Ignite and Exhaust are basically the same win rate, although ignite is more popular. Choose situationally.

You may follow the "Core Build" strictly if you like, but I would recommend adapting your build based on your team comp, your lane, and the enemy team. The highest win rate items are hypothetically the best if you only play one build.

Use the "Highest win rate items" section to adapt your build for the circumstances. That way even though you are adapting, you also know your build isn't bad.

Items that are expensive or only built when ahead (like Mejai's) are going to have higher win rates, since losing supports rarely buy or keep these items. You should still be willing to build them if the circumstance is right.