Dark Ranger is the most useful, but not strongest neutral hero.
First off, her skellies are as good as the bears in the late games, since she can make unlimited amount of them with black arrow.
Another thing, she can jus charm the monsters like Frost Wyrms in the late games. Plus, charm doesnt have long cooldown.
Also, her silence really pwns enemy's heros and casters. Think, when your fighting against human, whats the good of Priests and Sorceress without slow and healing? When you silence them, Human's meat shiled dies off, VERY fast.
However, the strongest hero, I would have to say Naga Seawitch. Her Cold Arrow is #1 Hero Catcher, and her Mana Shield helps her live and run away from the enemy when she is dying, and Her Forked Lightning rips the enemy's meat shield apart. Her Tornado really rocks too.