Well, I've been playing pantheon for a couple months now and his recent buff is amazing!!!. anyway I've looked up guides on mobafire and they're pretty good but the builds don't work for me. So this will give a armor penetration build that works very well in 1 V 1 situations (and can destroy tanks).

You must go greater quints and marks of desolation for a whopping 25 ArP which gives your spear a powerful bite even at level 1!
For seal's I go greater seal of evasion for it nice dodge speed that alone does not help to much but when combined with dodge masteries, ninja tabi and your passive can absolutely wreck your opponents
Glyphs are optional, you can go mana regen, AP or anything else, none of them help out to much. I chose focus so you can throw out a couple more spears.
So to wrap up:
3 Greater quint's of desolation
9 Greater marks of desolation
9 Greater seals of evasion
9 Greater glyphs of focus

21/9/0

I max out in attack:
Deadliness
Cripple
Alacrity
Sunder
Offensive masterie
Brute force
Lethality
Havoc
And 1 in sorcery or archmages savvy

And for defense:
Hardiness
Evasion
Nimbleness
Menders faith

Summoner spells: Heal and exhaust

For summoner spells the reason I chose them is heal always helps in combat situations and when your fleeing or fighting, unlike flash which is better for fleeing but cant really help in a fight... exhaust works great after the your stun wears off so you can chase the enemy down with your superior speed, which for pantheon is quite high

Well the first Item I get is chalice of harmony (by building and starting with meki pendant)
then ninja tabi OR if the other team has a lot of cc mercury's treads, now for the real items Black cleaver for its great damage and passive, then last whisper for its also great damage and passive, with those two items and runes you'll have 54 armor penetration after three hits and 40% armor penetration so your spear will go through anything!
after those I usually get frozen mallet so you have more health and for your final item you can get ghost blade. the reason? To ulti in and BD anything in your way may it be a turret or inhibitor and sometimes the nexus, your just standing along side the wall of the enemy base meanwhile at your base the whole team is just finishing up destroying the last inhibitor turret and if you go in to fight while the rest of your team is dead you will die to. The only path to victory is to destroy there vulnerable nexus, you ulti in activate ghostblade and destroy the nexus, saving your nexus with 100 hp and getting your team the victory.

If the game is long enough for you to get quite a bit of money and have all these items sell the chalice and get a infinity edge OR bloodthirster
Also if the opposing team has a karthus I usually rush a hexdrinker after chalice and boots for its early attack damage and passive to save you when your low health after a fight and karth feels like using his noobish ulti, wasnt nerfed and your ulti which does the same amount of damage at most and in one general area is nerfed so your range is now next to nothing so when you spawn you cant jump in and enter a team fight (or low levels using revive can die, revive with 400 extra hp then ulti in and finish there killer off)

First: Spear shot, Aegis, Heartseeker, Spear shot, Heartseeker, ulti, etc...
Pretty much get an early level 2 aegis and alternate between Heartseeker, spear shot and ulti until you cant level them up anymore, than level up Aegis

This isn't the nicest looking guide but it works! (try it out and if you think you have something better ill be happy to try it)