Does Kakashi, or Iruka have the best poker face?
Honestly, I think neither of them have a good poker face 
Kakashi thinks his poker face is good, since half his face is hidden by his mask, but because he wears that mask all the time, he’s never really had to hide his expressions before; and if you’re someone who has been around him long enough, like Iruka, you know exactly what to look for when he’s trying to hide something.
Iruka doesn’t do much better. He might be able to keep a blank face for a little bit, but when his emotions are high, he tends to slip up with little ‘yes!’s and sighs. And then when he remembers that he’s not supposed to give anything away, he’ll play it off as him just being teasing. Meanwhile, it’s very obvious how genuine his little exclamations are XD
Together, the two of them are like the equivalent of a toddler claiming to not have drawn on the walls, when they are covered in ink and are holding the markers behind their backs - guilty as charged!