You might want to breed it until you get a timid or modest nature porygon. Rash is okay, but not the best.
On to the moveset, I really like the nasty plot set I saw on this board not too long ago. It went something like this
Porygon-Z@Leftovers
'Download:downloads foe's best stats'
<Timid +speed -att>
evs: 6hp/252sp.att/252speed
-nasty plot
-tri attack
-thunderbolt
-ice beam
Porygon-Z has really high sp.att. After nasty plot it's just plain nasty. 'Adaptability' is a good ability too to give tri attack an extra boost, so it's your call.
Another one that people like goes like this
Porygon-Z@choice specs
'Adaptibility'
<Timid +speed -att>
evs:6hp/252sp.att/252speed
-hyper beam
-thunderbolt
-ice beam
-tri attack
This guy is set up that it can actually KO blissey in 2 hits(hyperbeam), even if it softboils(during the turn you recharge).