Which level 2 creature is the best?

Hmm... are harpies better at sieges than storm elementals? I'm surprised. How did that happen? :)
I already understand that you favor harpies; I, on the other hand, prefer storm elementals not only because of their stats, speed, and the fact that they are ranged attackers, but also because of their "usefulness." Let's look at your example of a castle siege:
So, you think that witch harpies are the most tactically advantageous in this example, right? And storm elementals can't defend the castle? You'll now say that if they fly up to the storm elementals, they won't be able to shoot, etc., but they don't have a penalty in melee combat, and not all harpies will even reach them..., plus, their retaliatory attack can be useful, but harpies lose at least 2 turns (a regular turn and another to get next to the storm elementals), and that can have consequences...

I always consider the maximum number of possibilities. Storm elementals tip the "scales of power" in their favor in the dispute with harpies.