I bought Pantheon back then when he was ULTRA OP and ppl just hate to see one on the enemy team, now after two big nerfs i thought, damn pantheonman is useless and unplayable now. But i was wrong, Riot intended to make pantheon more than a one skill depedant DPS champion, the nerfs forces you to play a pantheon who can actually fight melee in a fight dealing Burst Damage at first and then consistent DPS to carries while burst is on CD, then back to bursting again.

Other Data:
This guide was inspired by the 2 biggest pantheon's guides out there, the one with 3 bloodthirsters. And the one who plays offtank (warmogs)

Pros:
Great early game harrass skill, Spear Shot attacks the target doesnt require too much skill to use it correctly
Amazing AOE burst damage Heartseeker Strike
Has an early and mid game initiation skill Grand Skyfall
If played correctly Mr. Pantheon will block a lot of attacks in a fight Aegis Shield
Unstoppable force if fed
Spartan

Cons:
Used to be OP, now is underrated
People say Pantheon can't carry (we will prove them wrong)
Needs to solo lane, if Panth is underleveled he won't do ANYTHING during a fight
He is, what i call, a "smart champion" you have to know HOW and WHEN to use your ultimate
Team dependant
Item depandant
No reference to 300 in pantheon's emoticons wich sucks :)

Arpen, MP5, CDred Flat

Greater mark of Desolation Armor Penetration is basically what every AD carry should have
Greater Seal of Scaling Mana Regeneration Mana regeneration to always be able to cast something.
Greater Glyph of Cooldown Reduction CDR to re-burst, and flat because Pantheon gotta own early game.

When talking about quints i have some recomendation
greater quintessence of desolation more armon penetration
Greater Quintessence of Experience because u NEED to be ahead of everybody in the game, always be top lvl.
greater quintessence of strenght 7.65 AD on lvl 1 MIGHT sound noob to you, but combined with 100/115/130/145/160 % extradmg from every SPEAR shot you shoot its a big OUCH for harrasing
Personally i like to use the strenght ones because if i dont rape at lvl 3, i wont do it at lvl 6 and so

I go for 21/0/9 'cause u need everything in offense to help u burst. then the extra 9 i take it for utility cause i love the mana regeneration (you can also take it on Defense, you are on your own here)

Ill put you on a normal game and a ranked-game scenario to explain possibilites:

NORMAL GAME SCENARIO
In low level normal games people will just pick their favorite champ to play and its noob, it's not funny, ill skip this

Mid level normal games (15-28)
Now you will see people playing tanks, some of them taking junglers and 2 carries per game.
you will take the role of carryDPS. (Like Master Yi, Tryndamere, Evelynn, Shaco, Olaf, etc.)
so if there is no other dps you pick Pantheon.

Ranked games
Now Mr. Hoplite AKA Pantheon is not a common pick in ranked games specially because if you pick him too early you are getiing countered by a big hp nuke AKA Vladimir and you will not that big impact on the game as if you are last pick.

1. Grand Skyfall
2. Heartseeker Strike
3. Spear Shot
4. Aegis of Zeonia

Grand Skyfall even though pantheon's ulti becomes marginal at late game situation it is a great tool for taking down champions at their towers (posting pictures later), ulti on their tower, plus stun, it's OUCH, nowhere to run.
Heartseeker Strike big importance item at mid game, burst damage that eat carries at their weakest time in the game
Spear Shot the carry skill, maybe when you saw my guide at first you went "rly? pantheon with lvl 5 HS at lvl 13! WDF im closing this up" , well this skill scales 160% of attack damage in a 4 second cooldown (not counting yoyo's cdred, masteries and runes), great harrass skill at first, powerful damage mid/late game plus some CC with Frozen Mallet
Math says: (300/4 dps)*(1.6) = 120 damage per second that reset's your auto attack.
Aegis of Zeonia after the nerf that they gave panth, stun is only marginal, i take the second point at lvl 10, you can take more points if you want, is situational.

Time to build your own custom Spartan

1. I begin every game with Long Sword allows me to last hit with real damage, harrass the enemy with a decent and cosntant amount of AD. And a Health Potion because i have enough gold to buy one.

2. Sword of the Occult Kinda of a wierd second item, i grab these when im lvl 6-7, try to ult and get some kills to stack these.

3. The Brutalizer One of my favorite items in the whole game: CDR, Arpen and 25 AD at a low cost.

4. B. F. Sword When you have this item you should start getting your Heartseeker's Strike up to lvl 5 because only with +45 damage you will burst down people.

5.a. If you got Occult and you have enough stacks to burst down people get a Giant's Belt for hard survability against nukes.
5.b. If you think your survability is OK and you are bursting down the enemy with ease build the Phage so you can keep up
5.c If you don't got the Occult and your damage is still marginal get another B.F. Sword otherwise you won't be able to carry.

6. Youmuu's Ghostblade for the time you get this item, there are going to be teamfights on mid, on dragon, on enemy/ally jungle, so your burst will not be enough to stop the enenmy, thats why this is the WHEN you get your first DPS item, yoyo's will give you 3 things: More damage (for every purpose), some critical chance (wich is nice to have) and its active power that allows you to get more Aegis Protection during a fight, and play like a regular dps.

7. Frozen Mallet every thing is good with this item, the slow, the hp, and the damage. In my personal experience i would take this item over Warmog's Armor every time, i hate warmog's because even if it show like a tanky item, its a survabilty item cause health only counters nukes and AOE damage from tanks, but it wont save you from high crit Tryndamere and tough % hits Warwick with mallet and Spear Shot Pantheon can actually kite an enemy to wait for cooldowns.

8. Bloodthirster you know why :)

9. Zeke's Harbinger helps you with your dps role