Sorry folks Pantheon just can't burst damage/kill anymore! Heartseeker Strike has been too heavily nerfed and it just can't drop Champs like it used to. I have played Panth for a long time now and he was my first main character over a year ago. However, I stoped playing him for a while and when I did pick him back up I noticed a significant difference. In order to play him effectively post nerf you must use him like and assassin with a harassing Spear Shot and a useful Aegis of Zeonia stun to hold those weakened Champs in place. Panth has now morfed into an assassin champ with a little more defense than other assassin types. Still he's no tank and I don't think any off-tank Panth build is really viable.
That being said Pantheon is still a versatile champion and that is why I have not recommended a full inventory of items. Just like with any other champ, Pantheon's build should be heavily dependant on who he is up against. I have suggested a core build and from there you should look at the enemy team, see who’s fed and adjust accordingly.
I have been criticized for not adding armor penetration to this build. It’s true that armor penetration is crucial for dps especially when playing against tanks or heavily armored champs, however, some characters don’t build any armor at all in that case those runes would have pretty much been wasted. Mana on the other hand is always useful against any champ because Panth needs more mana all the time. The point of my runes and masteries is to give Panth useful buffs against any champ and then determine in game where he needs to go from there. Just keep that in mind at all times, there really is no one cure all bad a** build for Panth, or any other champ for that matter.

Pros:
Good DPS and Harassing ability (spear shot scales extremely high with AD)
He’s the number one Stunner!
Dodge and Passive blocks add to his defense
He has a personal teleport
With lifesteal he can kill any jungle creep he wants (endgame even the epic!)
Cons:
Low health (especially for a melee character)
Very low mana
Longer cooldowns

This build is not tanky at all and requires a fair amount of skill to play but the rewards are a staggering number of kills and near invincibility towards the end game. This build moves fast, attacks fast, hits hard and finishes the kill with slows and insane criticals. The point of this Panth is to harass with Spear Shot until the enemy is low enough health to burn them down. Then this Panth can rack up on kills by Aegis of Zeonia jumping the low health enemy and getting away quickly. He has enough defense to do this because I utilize his Aegis Protection innate passive block often by giving him some NOS attack speed. Furthermore this Panth utilizes a lot of dodge and has the passive mastery to increase his movement speed after a dodge. These passives are very effective because Panth is kind of squishy and doesn't have a lot of escapability. These passives provide him with good defense in the form of dodge and blocks and give him a decent chance to get a 10% move speed buff while running from an enemy. He doesn't have a lot of mana (though the runes make up for that) but I Rarely use Heartseeker Strike (E) until the end game, and even then only sparingly. It is necessary to get early however to get the guaranteed critical strike and finish off those fleeing champs or to come out with the kill if I'm in a team fight. (Hence the name of this build!) By not using Heartseeker Strike (E) you will save mana and be able to lane for much longer. The Heartseeker Strike (E) has been painfully nerfed and is not all that effective anyways but the passive is still really useful especially with an Infinity Edge. You can do more damage with your auto attack than the Heartseeker Strike (E) does over its duration if you can hit them more than twice anyways (you should be doing crits often). Overall this is a squishy but evasive dps build that can take a tank straight up and is able to kill a caster in a few seconds. 1v1 this character is unstoppable, 2v1 this character played skillfully may still win, but 3v1 just run!

The main ability for this build is Spear Shot (Q) however I get Aegis of Zeonia (W) first for the potential early game stun/team kill. Use Spear Shot (Q) early game to harass your enemy while laneing and then when they are half health and over extended jump on them with Aegis of Zeonia (W) do a quick Spear Shot (Q) and finish the kill with your critical damage. If they get the chance to run Spear Shot (Q) will usually finish the kill and the cooldown is low so I can usually get 2 off before they can reach a tower.
This Panth is great at tower diving for the kill because his passive Aegis Protection will block the first tower shot. If an enemy is next to their tower trying to (B) with less than half health, make sure your passive Aegis Protection is up and run in and Spear Shot (Q), if it doesn’t kill them Aegis of Zeonia (W) and run. This is possible because the Aegis of Zeonia (W) resets your passive Aegis Protection to block the next tower attack. So you get 2 tower shots time to kill the enemy before you need to get the heck out of dodge. Mastering this will get you so many free kills it’s hilarious.
I hate to say this but the ultimate Grand Skyfall sucks now. Yeah I know it does 1000+ damage and stuns the enemy, but who the heck would stay in the middle of the circle for 5 seconds, not to mention the time it takes for the circle to even show up. That’s not to say that it’s not useful though. You have to know how to time it but it can be great for catching an enemy running from your teammates. Try not to jump on top of his buddies coming to save him though! This is always a risk when using it this way. Look at the mia’s on the mini map and keep that in mind when jumping. Another use for it is to save a tower that is about to be captured. If you see an enemy laneing alone and about to push to a tower, use your ultimate Grand Skyfall BEFORE they get there and maybe (if your lucky) the enemy will stay and try to kill the tower with his minion buddies. Sometimes tanks must think “Oh, Panth can’t hurt me I got lots of health and minions to help out” but Grand Skyfall (R) will kill all the minions in the circle and stun the champ. That means there’s nothing left for the tower to shoot at but the stunned champ and we all know how that turns out. In ranked though forget about trying to land this on a champ, the wait time been too nerfed for that. In good games just use it to get around the map and save towers. I honestly use it more like a free Teleport than anything else.

As for Runes, I get a lot of dodge to make up some defense and to activate the mastery that gives 10% move speed after a dodge. This is not a significant boost but can come in handy often, especially when running away. I also go for attack speed because the more often you hit the more often you activate your Aegis Protection passive block. This again makes up for your lack of defense while adding some offense. Finally I go for extra mana because Pantheon has no mana and even spamming only spear shot can dry him out. A lot of people prefer mana regen to extra mana but I don’t just because I’d rather have the extra mana for the long fights where you may spam 3 or 4 Spear Shots and a couple Aegis of Zeonia stuns before it’s over. It’s really a preference thing though, I just like to get into the really big team fights mid to late game.

For masteries I go all attack and the leftovers go to defense. I have just enough points remaining to get the dodge speed buff because that little bit of speed has so often made the difference in me getting away instead of dying. I go all attack because I really build Panth as a DPS character, though he’s arguably not a pure DPS. The masteries reflect that strategy to give Panth as much offense as possible without making him as squishy and useless as a caster without any range.

For Summoner’s spells I go and
I really like Exhaust because it can help me chase down that champ with no health and critical him till he’s meatloaf. I also use it defensively to help me get away from that meanie that’s chasing me down.
I use Flash offensively and defensively as well. Really for the same reasons as I use Exhaust, to get away or catch up. The benefit of Flash is that you can jump over a wall and hit an unsuspecting target with a quick stun and spear and then just watch as he pees his pants wondering whether to stay or run from the guy who just put him at half health.
Summoner spells are always up to the user’s play style though. My only suggestion is that you get Exhaust because it’s the most useful thing to spend a point on in you offensive masteries to open up the next level.

Attack speed is crucial to my play style so I usually get a Dagger first then I go ahead and complete the Berserker's Greaves because Panth needs to be fast on his feet. Next I get an attack damage item usually the Pickaxe but if I have the money I go straight for the B. F. Sword (they both build the Infinity Edge anyway). After the first AD item I go for the Zeal to add more attack speed and some more very useful move speed. After the Zeal I will typically finish the Infinity Edge because the damage is great and it makes his criticals insanely powerful. Next is the Phantom Dancer because at this point my crits are hitting hard and often, and so attack speed and move speed is more useful than more damage. At this point you have the Berserker's Greaves, Infinity Edge and Phantom Dancer. That is really my core build. Everything else is merely suggestion however I highly recommend Bloodthirster and the Frozen Mallet. This build really benefits from the health and the passive slow of the Frozen Mallet. With the added lifesteal from Bloodthirster Pantheon truly becomes a God of War able to take ANY creep (even the epic) Solo! He just blocks so often and regenerates his health so quickly that no one can take him face to face. For my final items I typically go for health and lifesteal with as much added attack damage as I can get. However if things aren’t going your teams way, armor and magic resist may be more helpful.
If the other team has a caster that is just tearing everyone up grab a Hexdrinker a Wit's End or a Banshee's Veil to deal with those nukes. If on the other hand it is an insanely armored tank that is wearing everyone down get a Last Whisper or Black Cleaver or The Brutalizer to penetrate that armor. Often I see people getting really smart when I start to rape their team, and they all get Thornmail. If that’s the case just counter it by getting your lifesteal up to at least 30%. I recommend Zeke's Harbinger but Executioner's Calling is also a good choice. That negates the Thornmails reflection effect and ensures that you won’t kill yourself feeding off of your enemies.
As you can see I leave this build wide open to modification. Some may find this annoying as they want a perfect Panth build but honestly there is no perfect build. Pantheon's main use is to counter the other teams best players. He is a great anti-carry champion. Always Always Always build to defend against their most fed player or, if no one's been fed, build to counter the majority of the other team's champion types (melee dps, tank, AP caster, ect.). Doing this will make Pantheon a scary foe for the other team to run across and will ensure he has a viable job which helps the team.