Which level 2 creature is the best?

Nothing surprising.
Arrows are no match for storms.

Quite the contrary. Yes, they have fewer hit points and deal less damage, BUT... arrows fire twice. I wouldn't rush to conclude that storms are better.

Added 4 minutes later

I disagree with you about harpies; harpies are very effective and deserve to be in the top spots!

I'm not arguing that harpies are excellent fighters, especially when you have an air expert and use the speed boost. :D Then it's a lot of fun - you fly across the entire map, attacking anyone, don't receive any return fire, and even return to your side. :D But... fighters are fighters, and before the battle turns into close combat, archers would defeat harpies. Of course, it depends on what you value more, archers or second-tier fighters. And I won't even argue with you if you prefer second-tier fighters. It's simply impossible to argue because we have "different preference categories," which means the argument is pointless.