One of the best things about Porygon-Z is it's abilities, Download and Adaptability. Adaptability doubles the power of all STAB moves, and Download increases Porygon-Z's Attack or Special Attack, depending on what the foe's highest defense stat is. With Adaptability, the power of Porygon-Z's Tri-Attack is lethal. And it can either paralyze, freeze or burn if you get lucky. If Porygon-Z had better Speed, it would be a huge threat. How do we fix his Speed? Choice Scarf, of course! Thunderbolt and Ice Beam are obvious choices; it's just great to have that covered. The last move is the only place where any thought is required. Hidden Power Fighting, with high power, takes a bit of work to get. So, it's easier to get something else. Hidden Power Fighting would've taken down Ice, Steels, Darks and other Normal types. Your main choices aside from that are Psychic, Signal Beam, or Dark Pulse (Ghost pokemon). I'd personally pick Dark Pulse. Don't want any of those? Hyper Beam can do so much damage with STAB + Adaptability it's ridiculous. It can even take down Blisseys! The only problem is the recharge.