I prefer this build for Pantheon, as it works really good to me.

As you will see it is an Anti-car build, but when you stun and heartseeker someone he is dead, so you can die only one 2+ vs 1. This doesn't mean that you can't kill 2 in 1vs2, you stun the one with more damage and hit them both with Heartseeker.

Just read and see the statistic.

Marks: Armor penetrtion is the best against Tanks, like Ramus, Malpite, etc

Seals: I dont go for so many AS with items, so AS seals is really usefull.

Glyphs: I never buy item for mana or mana regen, so mana reg glygh is very good for Pantheon.

Quintessences: Armor Penetration is the most suitable on Pantheon

As always 21 to offence.
I don't choose 9 in defence as I play offensively and 9 masteries on defence will not help at all.
So i prefer 9 points in Utility

Full damage and ArP. Also a little AS and Lifsteal. You dont need anything else.

Start with Dorans Blade, or with Health or Mana regen.

In the middle game you can sell anything you buy as first item.

Remeber that your first item is nver on your build, is just to give you power in first 5 minutes

A lot of times I like go mid, so if you do so, I start with Spear shot for the last hit minions.

If you do not start mid, choose stun for fisrt blood.

I always go as much heartseeker as is possible, and then spear shot. In the end stun.

Ghost is the most important to escape or to support your team to push.

Exhaust it is very usefull on 1vs1 or to Exhaust someone to escape.

You can also try Flash insted of Exhaust, to escape musch easier/

Pros:
-Damage
-Crit cahnce
-Crit Damage
-Attack speed
-Lifesteal
-Armor Penetration

Cons:
-Anti-carry

I tested this build first time when i see a video with a really g-g-godlike Pantheon with 41 kills! I tested this, and it works.
Play Video
Hope help you.

Thanks for reading.

Sorry for bad English :)