Assuming if it's EV trained in speed, how would a choice scarf Modest Porygon Z stand up speed wise?
It'd do okay, but sticking to only one attack is a major suck. Also, CS Medicham and CS Heracross will crush it (assuming that they're Jolly Nature)
Next, Timid Porygon Z can outspeed Adamant Garchomp, therefore making it a good send-in if you anticipate the opponent's Garchomp has 303 or less Speed.
Now as far as movesets go, I can suggest this:
Porygon-Z @ Life Orb
Ability: Adaptability
EVs: 4 HP / 252 Spd / 252 SAtk
Timid Nature (+Spd, -Atk)
- Dark Pulse / Hyper Beam
- Thunderbolt
- Ice Beam
- Tri Attack
Basically just traditional typical sweeping. Hyper Beam if you want to go out with a bang.
Then there's pulling Agility, with Modest instead of Timid, and putting that over Dark Pulse. It's risky, but sinister if you don't want to run with Choice Scarf.