The only problem with Choice Scarf Porygon-Z is that it just can't do quite everything you need it to. Now the things that I personally think Porygon-Z (and most other pokemon) need to be an effective Choice Scarfer are as follows:
1.) Outspeed Scarchomp: Porygon-Z can do this, but only by running a +speed nature and a whole bunch of speed EV's. The downside is that you lose power. I'll adress this later.
2.) Type coverage: Lol, Porygon-Z is the the king of coverage.
3.) Ability to KO other leads: This is where Porygon-Z drops a little. Even with 135 base SpAtk and great type coverage, Porygon-Z can't OHKO the majority of the pokemon in the top 10 list. For example:
A Modest Porygon-Z with a Choice Scarf has a 75% chance of OHKO'ing a 4 HP / 0 SpDef Gengar with Dark Pulse. A Timid Porygon-Z (needed to outspeed Garchomp) only has a 18.75% chance of OHKO'ing the same Gengar. Weavile will be 2HKO'd by Tri-Attack either way. If you run HP Fighting, Weavile will be OHKO'd. Tyranitar however, is only 2HKO'd by HP Fighting thanks to it's SpDef boost in SS. Infernape is also 2HKO'd by an Adaptability Tri-Attack from a Timid Porygon-Z. Are you beginning to see the problem?
Porygon-Z deals a lot of damage to a lot of stuff, but doesn't OHKO much without Nasty Plot or Choice Specs. The solution? Download.
Most of the pokemon on the top 10 lead list will have Def > SpDef, meaning Porygon-Z gets an auto 1.5x SpAtk boost immediatly. Only the following pokemon will not give Porygon-Z a SpAtk boost:
Gengar
Weavile
Bronzong (depends)
Abomasnow
Also, Infernape and Salamence are toss-ups, since they have equal Def and SpDef. Salamence is OHKO'd by Ice Beam anyways, only Infernape isn't OHKO'd without the boost (assuming you run Timid). IMO, Download, while always thought to be the inferior ability for Porygon-Z, may very well be worthwhile in a lead, since it gives Porygon-Z the power it needs while being able to OHKO just about everything on the top 10 list.
If you want to go beyond the top 10 list, Download still seems viable. Roughly half of the top 20 leads will give Porygon-Z a SpAtk boost, and you have a 50% chance of getting the boost on 4 others (Ambipom, Salamence, Infernape and Crobat)
The last issue is HP Fighting. HP Fighting is the only way to have a chance at OHKO'ing Tyranitar (even then it's not a 100% KO, and Boah has no chance at being OHKO'd) and Weavile (since it doesn't give you a SpAtk boost with Download) However, this means you must make a sacrifice to Porygon-Z's moveset. My opionion? Thunderbolt.
The only thing worth TBolting is Gyarados, and that's so obvious that Gyarados isn't going to stay in to take it. So, if you wanted to use a Choice Scarf Porygon-Z as an anti-lead, here is what I would suggest:
Porygon-Z@Choice Scarf
Timid
8 HP / 252 SpAtk / 248 Spd
Download
-Tri Attack
-Ice Beam
-Dark Pulse
-HP Fighting
This is a standard set, but effective nontheless. I am now going to go back and re-read my post to make sure that it makes sense.