There's no denying that the only real hero in the original trilogy was R2-D2, and certainly one of the more heroic ones in the prequel trilogy as well!
He was pretty much the only character who did what he did for heroic reasons. He didn't whine like a bitch at every turn (sure, he showed concern and worry with his sad beeps, but that's not whining), he bravely charged into the fray when he had to despite the dangers to himself time and time again, and without him the other protagonists would never have gotten as far as they did.
It's a shame he doesn't get as much credit as he deserves in that regard.
B8 isn't that far behind either, though his AI is clearly more of a manipulative sort. He knows that he's cute and adorable, and he uses that fact to get other people to help him do the dirty work. Which is, from a writer's point of view anyway, the better way to go about it rather than having him be the hero of the story.