This guide is based on the data at http://aram.lolalytics.com/champion/Alistar/. Last updated in patch 7.22. The data changes, so this guide may be a bit inaccurate when you find it.

The core build is jut an example. There are a lot of really good situational items on Alistar. Adapt your build to the circumstances. The items by win rate section is to give you an easy way to evaluate your options and deviate from the standard build path.