Well, that would be completely up to you, but I would say
Trying to find a/the perfect Porygon to be a Porygon-Z is defenetly worth it. However, that would take you quite some while, and it's hard trying to find some of these guys.
The Mild Finicky one is good, but a Modest Mischievous is much more powerful and better, depending on how the IVs and EVs are set for them.
(take Astro's advice, breed! But it could take some time, so searching for them is a better option.)
So I would recommend the Modest Mischievous Porygon to be the Porygon-Z, but I might take. While, so happy hunting for one!