I have played Pantheon extensively and have tried to build him in all the ways possible. After a lot of research and playing and winning many games with Pantheon, I have come up with this guide to help the Pantheon players out there. I will be suggesting the beat way to build him.

For runes I use:

9 X Greater Mark of Desolation
9 X Greater Seal of Scaling Mana Regeneration
9 X Greater Glyph of Cooldown Reduction
9 X Greater Quintessence of Desolation

This provides the best set for Pantheon. The runes work perfectly in combination with the Item Build and Masteries that I have suggested for him.

With the onset of the new masteries I take the 21-0-9 path to get most damage along with mana regeneration.

Start with Doran's Blade to give 100 health, 10 attack damage and 3 lifesteal. It makes you stay in lane longer.

Stacking 2 Doran's Blade is a really good idea on Pantheon as it gives him good survivability and attack damage initially.

Then take Boots.

Try getting The Brutalizer next before completing boots.

The best pair of boots on Pantheon is Mercury Treads as it gives the most required magic resistance and tenacity. I have seen many players making Berserkers Greaves on Pantheon. Its a really bad idea as playing Pantheon as an auto attacking DPS character is not good. He should be played as a melee caster.

Then try getting Bloodthirster. Charge it up by farming as a fully charged Bloodthirster is the most AD giving item.

After this you need survivability. Believe me when I say that we do not need any more attacking items on Pantheon as he becomes a pure glass cannon. If we keep on making offensive items, maybe he will get a kill in team battles but will die soon too.

Next get a Warmog's Armor. It is the best Health providing item in the game and is most needed on Pantheon.

Then but Atma's Impaler as it gives armor, critical chance and also attack damage (2% of total health). It will be giving you around 60 Attack Damage with your health. This Attack Damage is not included in the stats above.

Then get a Banshee's Veil to complete the build. It gives most required magic resistance and a great passive.

With this build if you play well you are going to rock. You will be the biggest killer for your team and it will be very difficult to focus and kill you.

Put your first point in Spear Shot and try to last hit minions or harass enemies in your lane. Then put 1 point each in Heartseeker Strike and Aegis of Zeonia to complete to complete your combo. Then try maximizing Hearteeker strike first.

The priority is as under:
Grand Skyfall > Heartseeker Strike > Spearshot > Aegis of Zeonia

For summoner spells I take Flash and Ignite. Flash is one of the best Summoner spells as it can be used both as an attacking and a defensive spell.

I use ignite to finish off enemies left on low health after my W-E-Q combo.

In the initial laning phase try to last hit minions with your basic attack or using Spearshot. once you have your W-E-Q combo ready try to find a suitable time when the enemy has overextended and use all three skills on him. You are very much likely to get the kill or maybe leave the enemy at very low health. Use ignite if the enemy is left at very low health.

Gold is the most important thing in the game and the most important thing is how quickly you are earning it. So go for a kill when you are sure of it otherwise concentrate on last hitting minions.

Try to stay in lane atleast till level 6 so that on first time back you can directly ult in a lane which is being pushed by enemies.

Go with the Item Sequence mentioned and DO NOT PLAY HIM AS AN AUTOATTACKER.

Move in and out of battle. Initiate the battle with Aegis of Zeonia, use Heartseeker strike and Spearshot and then move out of the battle. Again go in and use this combo. Stay in battle only if you are sure that you wont die. This way you will be churning out pretty good damage in bursts and thats the way to play an assassin. Practice landing the W-E-Q combo as fast as possible.

Always focus the enemy squishies in battle. If you perform well you will be the first one to get focussed by the enemy team. So the build gives that survivability that makes Pantheon play superbly end game too.

You will also have to learn to land the ult 'Grand SkyFall' properly. Try to always enter team batlles with 'Grand Skyfall'. If it is landed properly it can decide the fate of a team battle.

Maintain all the necessary common sense required for any chars that you play like using wards and not jumping into multiple enemies alone etc.

Try out this build and provide comments on how this build works. I am open to any type of discussion and please provide comments on how can I improve this guide.

Thanks a lot to all the readers.

I have seen that the views on this guide is increasing a lot but people are not putting comments and votes. Please try out this build and then give comments and votes. And please don't spoil the guide by giving a down vote with some wrong reason and then not returning back to listen to the explanation.