Which level 2 creature is the best?

As with all discussions about units of the same level, it's difficult to choose which one is better, just like comparing an archer and a warrior – they are DIFFERENT. But still, I would highlight the "storm" unit :-). It has very powerful attack for a level 2 unit. I don't know if one shot from a storm is as good as two shots from an archer, but in close combat, the archer simply starts to disappear before your eyes, unlike the storm – and that's a significant advantage for the elemental. So, I'm for the storm. For defending a castle, I would highly recommend battle dwarves; they are very durable and like to defend against magic, and it's not easy to run away from them, and it's not so easy to take a castle defended by a good stack of dwarves, especially if there are also dryads.