I agree that Othello is more suspicious of human nature than Iago is. It's very clear that Othello is primarily a warrior; he explains that he is "rude in speech" and lacks the softness that many other men posess because of his experiences in battle. One might assume someone who is able to rise to the position of General in the Venetian Army is shrewd and lives in a constant state of alert.
Iago, as kapokkid notes, does seem to be distrusting of everyone. However, there IS something about him that makes other characters believe what he says. He is able to appear sympathetic and kind, and everyone around him thinks him to be "honest" and a true friend. Instead of "suspicious of human nature," I think it's more accurate to simply describe him as evil, as there is no explanation given in the play for his extreme behavior. (Yes, he is mad that he didn't get the lieutenancy. Yes, he suspects that Othello has slept with Emilia. But none of these complaints justify the numerous lives he ruins or the enjoyment he gets from doing so.)
My first guess would have been Iago as he appears to distrust everyone, but on second thought, Iago also appears to trust human nature in a way that Othello cannot. Iago knows how to help Othello start to suspect that something is happening between Desdemona and Cassio. Iago appears to trust the way that Rodrigo will react to certain stimuli as well, so too with Cassio and Desdemona, he is absolutely certain that he can manipulate them at will.
Othello on the other hand is so ready to distrust Desdemona and her love for him or Cassio and his loyalty to his commanding officer and friend. He trusts Iago but no one else once he has gotten to Cyprus.
So in this case, I will agree with the previous post: Othello.