I think all dogs can be trained, and it isn't a matter of whether they know what they are supposed to do, but whether they want to do it, thank you very much. lol

I mean, a cat is very intelligent but if you tell it to do something...they either give you a blank look or do the exact opposite! LOL
But I think that the herding/working breeds are among the "easiest" to train. (When I say easy, I mean they pick up on things and actually preform them for you, if you can handle that much dog). Like Border Collies, German Shepherds, Schnauzers, Belgian Shepherds, Labs, Goldens, Poodles...
But just because a dog isn't very trainable doesn't mean it isn't smart! My dog, Rose, had figured out how to bust out of her kennel multiple ways, but she is a more difficult dog to train. She knows what I want her to do, but if she doesn't feel like doing it, she won't. That being said, she knows when I mean business and she listens when it counts.