Gollum. He's not the brightest at battles because he was a Hobbit and then he lived underground for five hundred years, so he doesn't have that knowledge.
Saruman. He probably pays his agents better than does Sauron.
Probably Saruman because he didn't have the worst of intentions, you could feasibly stand up to him with some difficulty, and he actually seems like sort of a nice guy a little bit.
The goblins aren't the best social butterflies so I'd probably go with them