[thread=20486]http://www.smogon.com/forums/showthread.php?t=20486[/thread]
Porygon-Z is in a tough spot in terms of speed where it's not outspeeding much of importance when it maxes it out, with nature and everything. Thus I would suggest modest.
Just the reason of outspeeding Modest/Adamant Salamence is enough to use Timid, IMO.
Also you'll go faster than any Heracross unless they're Choice Scarfed.