This is my current build for pantheon. I have played many games with him and have tested out many different builds. I am not trying to brag to much but I have yet to find another pantheon that can defeat me. I also win 1v1 battles as long as I am not versing a champion who has been fed. almost every game i have the highest kill count and usually in the top 2 for most assist. I find it fair to say that this build works.

-High DPS (its your job to do damage!)
-Great carry and pusher of lanes (your going to make alot of gold with this build. after 1 B.F sword you kill minions with one Heartseeker!)
-survivability (at least for a pantheon)

-early Mana Starvation

Flash- Never leave home without it. great for getting out of danger or finishing off your opponent. This is a multi purpose summoner spell and both a offensive and defensive ability for Pantheon.

Exhaust- Again this can be used offensively or defensively. stop your runners and your chasers. Also having the Cripple mastery increases the pain on your target.

notable subsitutes- Heal is a good one. keeps you in the fight longer and suprises your enemy when they think there going to finish you off. Also since your more then likely going to be having a laneing partner, might save his hide too.

21-6-3. Your a melee DPS, you better make sure you do your job. my other materies are for extra armor, magic resist, and health/mana regeneration. It doesn't hurt to live longer.

Spear Shot
Great harassment tool and great at finishing off anyone that is trying to run from you. although spear shot is a great skill, you will want to master this after you finished mastering Heartseeker. I do see some spear based pantheons out there but since the range of spear shot is only 150, more then likely someone has better range then you do.

Aegis of Zeonia
this ability is a great start to your slicing and deicing. but only put one point in it! finish the ability at the end. why? because rather it is at level 1 or level 5, it will always be a 1 second stun. the different between level 1 and level 5 is only 100+ damage and 10 second lower cool downs. this ability is also useful to stun someone who is chasing you.

Heartseeker Strike
This is pantheons bread and butter. most of your damage is going to originate from this ability. Master this first and thank me later.

Grand Skyfall
Since the latest patch, skyfall now has a sorter cooldown and casting time! which means that you will actually be hitting people with your ult now!

Starting items- 
Long Sword
Health Potion x1

This is nagotiable but I start with a long sword (cause it builds into many items in the future for you) and a potion.

Early game Items-
Brutalizer
B.F Sword
Boots (Berserker Greaves or Mercury Treads)

The order of these three is up to you. but damage is your number one goal here. usually i get the brutalizer or B.F sword if im getting the kills fast enough.
B.F sword is just nice raw damage but the brutalizer is nice for the cool down reduction and the extra armor penetration.
for boots I usually go Berserker Greaves or Mercury Treads. if I am up against magic users or allot of stuns/slows/taunts etc, then i go merc boots. otherwise, I go Berserker boots for the extra attack speed.

Mid Game Items-
Last whisper OR Black cleaver OR Madred's Bloodrazor
BloodThirster
Frozen Mallet

By mid game your enemies have more HP, Armor, and do more damage. Last whisper is good against high armored heroes and Bloodrazor is good against high HP heroes, and both give damage and all the attack speed ya need. Bloodthirster has the highest damage if maxed out, and since you can kill waves of minions in one heartseeker, you will have this built to max in a matter of minutes. BloodThirster also gives ya all the life-steal you will need. The frozen Mallet is mainly for survival. With +700 hp your not going to see death often. Frozen mallet is also nice to reduce movement speed of your enemies. No one gets away from you now!

As you can see all these items increase damage. Remember, damage is your number one concern. Your second concern is survivability. with tons of extra HP and some nice life-steal, your not going to go down easily.

Late game Items-
what ever you want!

By late game you should be about finished with the other items I have mentioned. I like to leave my last slot open to what ever is needed for the current fight. However, Most of the time i never make it to this far because the game has ended or the other team has surrendered. Since every fight is different, a good player needs to learn to adjust to each situation while still doing your job. Here is a list of items that have benefited me for different situations:

Infinity Edge- Pure damage. your crits are now as dangerous as your Heartseeker. Have fun laughing at anyone who opposes you.
Banshees Veil- extra HP,mana,and magic resist. Great against a team of casters.
Thorn mail- tons of armor and great against melee.
Warmog's Armor- Wanna be a tank now? take this.
Wit's End- i believe this to be very underrated by most. but removing mana from your enemies and turning it into damage with 40% extra attack speed is note worthy.