The top champions not in lane to counter Alistar include Samira, Tristana, and Lucian. These champions are all great at countering Alistar with decent win rates over 56%. Even if these Alistar counter picks are not taken in your lane, it's not a good idea to choose him into any of these matchups. They will most often crush Alistar.
Yet, if the enemy team choses either Swain or Ivern, you should seriously consider selecting him, as he easily dominates these champions with win rates over 55% against both ofthem.