Billy could fry Ultron's inner workings. Well the legacy version at least, but these days Pym has been fused to what may be a superior version of Ultron. I'm not sure if I remember correctly, but wasn't the most current version of Ultron melted or some such? Billy's speed is also a huge factor. If I were to choose which of these guys that I'd be in a fight, I'd choose Billy, as that speed would afford him to pick his shots with minimal chances of Ultron being able to counter them.