Hello Moba, Jericho Here, putting out the first build i made, and the first one that's been working since the day pantheon came out, though i miss the days of madred/cleaver HSS. My build won't make you a behemoth, this just happens to be the way i play him and it works for me, test it, you might be surprised.

Pros:
He's a great AD carry.
He has great damage all game thanks to the new magic meta-game.
He's very versatile, his builds don't need a lot of kills as long as you can farm.
He's a very good harasser.
He can be used as a chaser do to his stun's long reach.

Cons:
He has a very squishy early game.
He requires skills to be executed in a certain order.
You need a competent team in order to pull out a truly excellent damage allocation.

My Runes are basically set up to run as much Armor pen that the game will allow me.

They are as follows:
Greater Mark of Desolation: A really good rune for AD champions like pantheon. it basically is the bread and butter of the rune page.

Greater Seal of Vitality: Some People like dodge seals, i prefer HP per level seals, why, because late game your gonna want that extra 175 HP over that 6% dodge, since unless the entire opposing team is AD and you actually get the chance to use ninja tabi.

Greater Glyph of Intellect: i actually used to run AP per level glyphs, but the more i thought about it mana is just a lot more useful, it ends up saving you in a variety of situations, especially if your chasing someone down and realize you only need *insert low amount of mana here* to throw that 400 damage spear shot and end that katarina's killing spree.

Greater Quintessence of Desolation: As with the Armor Pen Marks, this is all about pulling the most damage out of this champion that you can.

Alright as for how i play this build the entire way through:

Doran's Blade/Doran's Shield: great starting item for some hp and damage if you choose the blade, or survivability if you choose the shield.

Last Whisper: the bread and butter of the armor pen family, this one weapon makes your HSS a living nightmare for anyone within range. the attack damage it adds is amazing and the passive is synergistic with pantheon's physical spells.

Guinsoo's RageBlade: The Attack Damage is a plus and so is the base AP, for those of you that don't pay attention, yes pantheon has ap ratios, and yes they are 1/1 damage conversion, why pass up an item like this, the passive is even synergistic with HSS, and you receive about 115 ap just from this item, the attack speed helps you bash towers mid to late game, and early game as well if you acquire it in time.If you don't like this item, or want an alternative a Hextech GunBlade does the trick as well, supplying AD and AP and a not to shabby active as well.

The Chalice of Harmony: Remember what i said about Mana Earlier, pantheon has a shallow Mana pool for a AD champ, and AD champs generally don't have large ones, so to say the least your going to need Mana. I like the Chalice because of the passive, but it also adds a bit of magic res, which works well with pantheon as most teams use him as an anti-caster. An Alternative to this would be a Manamune, gives you Mana, and the conversion passive makes you that much more deadly, it's a toss up, 15 more ad and 1.4k Mana, or Mana Regen and magic res, the choice is yours.

The Hexdrinker: A recent addition to the League's weapon vault, after being nerfed considerably in the test realm, is a very good anit-caster AD Weapon, 35 AD, 35 Magic Res, and a passive that blocks 300 magic damage when you drop below 30%. This Saves me a bit in mid game, but late game most enemies have a large chunk of burst so it's really just for some AD, perhaps switching for a bilgewater cutlass, giving yourself another active to cast which can be built into a second gunblade, or a youmou's ghostblade would be more effective against teams that don't stack armor or very little armor, or if you'd like a bit more armor pen.

Ionian Boots Of Lucidity: the first new set of boots in a while, thanks to a certain udyr, these dandies give 15% cool down reduction, paired with blue buff, your never going to run out of mana and you are a killing machine, these of course can be traded out for ninja tabi or mercury treads if your dealing with Crowd Control.

The Black Cleaver: Perhaps one of the better parts of this build, adding a generous 55 AD, and a passive that tears enemies to shreds, this beauty is my favorite item to use with AD carries. Don't want the attack speed and want life steal? switch it out for a bloodthirster.

Exhaust: Perhaps more useful then it was before, this is used to slow down your enemy, set up flanks, and used for tough get aways. as stated in the last patch notes, this is no longer a blind, but cuts down physical damage by 70%, and spells by 35%.

Flash: this skill is fun to use, it's an instant blink if you didn't already know, useful for several methods of ganks and get aways, i personally use if for catching an enemy with Flash and Bash, which is flashing in and immediately using your aegis of Zeonia, or if you think the enemy is just out of range and they're at 1/3 HP, spear throw, flash and autoattack, this procs the damage from spear shot and the auto attack damage, dealing an instant death to the enemy.

Spells:

Spear Shot: Used for harrasmant and last hits, doesn't proc critical damage, but has a nice Damage ratio.16 / 28 / 40 / 52 / 64 base damage, and 100 / 115 / 130 / 145 / 160 % of your Attack Damage is applied

Aegis of Zeonia: A stun, that moves pantheon to the target champion, has a 1/1 AP ratio. 50 / 75 / 100 / 125 / 150 magic damage and 1 / 1.25 / 1.5 / 1.75 / 2 second(s) stun duration.

Heartseeker's Strike: Pantheon's main damage dealing skill, has a base of 4 / 6 / 8 / 10 / 12 Attack Damage, and applies 20 / 25 / 30 / 35 / 40 % of your attack damage, the damage is doubled against champions.

Grand Skyfall: A Global Ultimate that teleports pantheon to a selected location after channeling, dealing 400 / 700 / 1000 magic damage. Has a 1/1 Ap Ratio.

Early Game your going to want to stay back and last hit minions, at level 3 if your enemy is not beefy you and your lane partner can try a gank. the goal here is to get as much gold as possible without getting zoned.