wakusei
I don't really see Jack as being very easily manipulated at all, actually.
The most easily manipulated was Locke, and that got him killed and turned into a new person suit for MIB.
MIB is clearly playing those around him for easy recruitment. Playing on their weaknesses. Claire didn't have Aaron, so MIB told her where Aaron was and gave her hate to grow inside her.
Sayid was broken, so MIB played to his guilt.
Sawyer lost Juliet, so MIB offered an escape.
Jack has grown in a positive way. And that was with Jacob's guidance. That makes sense, I don't at all see how the Claire manipulation could have been positive.
I'm not sure who resurrected Sayid, but whoever did was definitely doing something malicious.
I wish I remembered the scene with the balance. When MIB took Sawyer to the cave with the names on it, he picked a rock off it and threw it. How was the balance to begin with? (I remember it wasn't balanced properly, as in one rock did not equal a second rock) What color rock did he pick off it and throw? How was it balanced after that? Were there 2 rocks or only 1?
But I would have to say that Jacob is also being manipulative, although we know much less about the consequences of his manipulation aside from that he wants them away from MIB (who may or may not kill them, he doesn't seem to be killing them).