I love how people were complaining about sinnoh stones being so hard to get, now we have more than we really want. Not really relevant to the question, but just fun to see
As for the question, would say probably dark, dragon, ice, or ghost would be best, assuming this if for PvP, as those are effective against giratina, and to a lesser extent latios (ice and dragon being effective on flygon) all of which are common in ultra league (however all of pory gon's moves are resisted by giratina, flygon triple resists Zapcannon and latios resists solarbeam and zapcannon). No hidden power type really is best for Porygon Z, as each would let it preform a different role. What really seems to hold porygon back would be its charge moves.
Psychic hidden power might be good if we see pokemon like tentacruel or other poison types more in PvP, but psychic doesnt have a very good effectiveness profile to begin with.
In PvP, Hidden power has higher DPT, while charge beam has higher EPT, with hidden power having a hair better EPTxDPT than charge beam.
