So i reckon i finished singles testing for now, so i'm going to sum up some conclusions. Offensively, the attacks worth having are:
ice beam/blizzard (ice) - super effective coverage against dragon, flying, grass, ground, ineffective against fire, ice, steel, water, can cause freezing
thunderbolt/thunder (electric) - super effective coverage against flying, water, ineffective against dragon, electric, grass, ground and quite a few pokemon with lightning rod, can cause paralysis
dark pulse (dark) - super effective against ghost, psychic, inefective against dark, fairy, fight, can cause flinching
Psychic - super effective against poison and fighting, inefective against dark, psychic, steel, lowers Sp-Def
Psyschock - like above, but with the important nuance that it's strong against special defense walls since it targets defense
shadow ball (ghost) - super effective against ghost psychic, ineffective against dark and normal
signal beam (bug) - super effective against dark, grass, psychic, ineffective against nearly everything else, can cause confusion
Ice beam is must have simply because so many pokemon are weak to it and so few outright resist it.
In the first slot (to which type you'll convert to) you can only really have three attacks: thunderbolt, shadow ball and psyschock. Thunderbolt because electric types have good coverage and there aren't STAB priority ground attacks, Shadow ball because fighting and normal type immunity and it won't be extra punished by the omnipresent earthquake, Psyshock mainly for surprise value since it turns a special sweeper into a a physical one, and you can cover for it with signal beam, dark pulse or shadow ball. Putting anything else in the first slot will get you killed very quickly.
Ghost and Electric porygon-zs can afford to run only two attacks, thunderbolt/shadow ball and ice beam, while leaving the third slot with a support move like recover. A psychic porygon-Z must have three different attacks for coverage, although having thunderbolt, ice beam and shadow ball/dark pulse also works well.
Because to get dangerous porygon-Z needs at least one free turn to be able to use conversion having setup moves like agility or nasty plot is not a good idea simply because in those two turn turns porygon-z is most likely dead and with two turns of setting up other pokemon would be equally, if not more deadly. Another thing to watch out is poison; while a porygon-z with thunderbolt or psyschock can OHKO the likes of toxapex easily, if the pokemon you switch in has toxic or is able to survive the first attack and poison you (like alolan muk, chansey, blissey), you're probably not gonna be able to take down even two pokemon.
Keeping that in mind porygon-z can be quite devastating if the opponent makes a mistake or has a team remaining that doesn't outright resist it, since without very good special defense stats most pokemon get OHKO'd by neutral damage. It is specially so if it is supported by a team that can compensate for it's weak spots and/or make it's good points even better.