This guide is designed to tell you the power of Pantheon. It debates whether to use attack speed to power up his shield ASAP or whether to go AD/offtank and use the block as an added bonus.

Pros
High health and AD late game
very,very hard to kill because of high hp and lifesteal rate
decent shield charging rate
slow means once you jump to them their goin nowhere fast
no need to fear AP champions because of high health

Cons
high health makes more vulnerable to items like madreds bloodrazor and deathfire grasp
damage mostly relies upon bloodthirsters so if you die you damage will have a significant decrease as will your lifesteal

Pros
Very high attack speed means not only high damage output but also incredible amount of blocks you can bring up
annoying as hell to lane agaisnt as he blocks almost everything
hgiher damage than AD

Cons
weaker vs AP carries as he cannot block spells so will die faster

Here i will explain the purpose of each item:

Dorans Sword
Always a good starting item with Pantheon or indeed any AD character. Gives a decent bonus to AD, health and 3% lifesteal. If you need a quick advantage over your lane opponent you could get another Dorans sword before level 10, then the bonus' become to small to be significant.

Frozen Mallet
This item not only gives more hp (and hence more AD because of atmas) but also a small amount of AD and a slow which will make chasing very easy
This item should be built ASAP after a quick stop for boots

Berserkers Greeves
Bonus movement speed and attack speed for more shield charges and damage output

Atmas Impaler
Gives a good AD bonus depending on how much health you have
With a fully stacked Warmog this should be around 80 additional AD!
also gives bonus armour and crit rating which are useful as well

Warmogs Armour
Gives a huge bonus to health and HP5 which increases as you farm! This item will make you almost impossible to kill

Bloodthirster
This item is very powerful as it gives you a huge bonus to AD and lifesteal which increases as you kill things, however if you die it resets the bonus' which can greatly reduce your damage and survivability

Other good items

Infinity edge
this gives a stable bonus to your attack damage (80) and bonus crit rating. But its main purpose is to give 50% additional crit damage, combined with your passive from heartseeker strike this could mean you take champions down with incredible ease when they are on low health

Sunfire cape
This gives a nice bonus to health and armour but also deals a small amount of magic damage to all around you every second! This could greatly increase your farming potential as well as your survivability

Manamune
This item is worth getting if you find yourself running out of mana a lot. It gives 350 bonus mana which increases everytime you basic attack or use a move. This will allow you to use your moves more often as you have less fear of going out of mana, it also increases your AD by 20+2% of you maximum mana which can be useful for more damage as well.

Dorans Sword
Always a good starting item with Pantheon or indeed any AD character. Gives a decent bonus to AD, health and 3% lifesteal. If you need a quick advantage over your lane opponent you could get another Dorans sword before level 10, then the bonus' become to small to be significant.

Madreds Bloodrazors
These are very useful items, not only do they give him a large boost to AS but also an AD and armour boost. Finally their passive deals bonus magic damage to target based on their maximum health which makes Pantheon slightly more effective at killing beefy tanks.

Berserker Greeves
same as above, movement and AS

Phantom Dancer
This item gives a huge bonus to critical chance, AS and movement for only around 2800 gold, a very good item for any AS champion

Black Cleaver
Gives a healthy AD bonus of 55 and 30% additionaly attack speed. Then it has an armour reducing de-buff, which stacks!
having high attack speed will apply the maximum effect of this de-buff in a very short time and make the target exceptionally easy to kill.
Tanks beware!

Zekes Herald
gives a small bonus to hp which lets pantheon survive a little longer as well as giving more AS (therefore more damage and blocks) and some lifesteal for survivability. The cooldown reduction means you throw more spears as well, always a plus as it charges up pantheon's passive even more. This item is very useful, you should get vampiric sceptre early and build into the herald late game.

Infinity Edge
this gives a stable bonus to your attack damage (80) and bonus crit rating. But its main purpose is to give 50% additional crit damage, combined with your passive from heartseeker strike this could mean you take champions down with incredible ease when they are on low health
This is better for this build than the other because of phantom dancer giving more crits than normal, also with more AS means more of those lovely big numbers than with AD/offtank pantheon.

Other good items
Frozen Mallet
Gives a nice health boost and a small AD bonus but its main purpose is the slow, if you find you cannot quite catch an opponent once you jump them because maybe they have ghost or similar then take this. Frozen mallet is also good if you find yourself being focussed more than you'd like to be and with 700 extra hp this will take you to nearly 3000, not very easy to nuke down.

Ionic Spark
This item gives a little health boost (same as zekes herald) but a huge 50% boost to attack speed and shoots out cool lightning blasts every 4th attack, so basically when you get a block they take more dmg. Very fun item and it is rather cheap at only 2300, however not as good as something like Black Cleaver or Zekes Herald.

Manamune
This item is worth getting if you find yourself running out of mana a lot. It gives 350 bonus mana which increases everytime you basic attack or use a move. This will allow you to use your moves more often as you have less fear of going out of mana, it also increases your AD by 20+2% of you maximum mana which can be useful for more damage as well. This will also charge up faster than the other build because of your significantly greater AS.