The best champions out of lane to counter Alistar include Samira, Tristana, and Lucian. These picks are all great at countering Alistar with high win rates over 56%. Even if these Alistar counter picks are not used in the same position as you, it's not smart to play him into one of these matchups. They will most often win against Alistar.
Conversely, if the other team picks either Swain or Ivern, Alistar mains should strongly consider selecting him, as he easily wins against these selections with win rates over 55% against each ofthem.