Not Updated For Current Season
This build is designed to maximize Pantheon's strengths while still covering some weaknesses pretty well. Obviously his strengths are his powerful abilities that grow with attack damage... and they scale WELL. His only problem is that he dies very easily, especially considering how little health he gets, and his lack of an escape ability. There are a few things built in to this build to make him a bit harder to kill, though, and unless you jump into a mess of CC or DPS, he should be faring pretty well.
The first things you want are a Long Sword and a Health Potion. The Long Sword gives you some good attack damage early in the game, which is more helpful than you realize. Normally I'd suggest Doran's Blade because it offers a noticeable amount of health and lifesteal and just a little less damage than the Long Sword, but the Long Sword builds into your other items whereas Doran's Blade doesn't build into anything.
You'll be building the Long Sword into a Brutalizer. If you're not doing so well defensively then get the boots first, otherwise get The Brutalizer. It may not have a ton of attack damage, but the cooldown reduction and armor penetration make it quite amazing for it's cost. You could instead go for a Sword of the Occult if you feel that you will play well enough for it to be useful, but I don't like to take chances like that. I almost always take the Brutalizer.
For boots I've chosen Ninja Tabi. Previously I took Boots of Swiftness for more speed so I can get away easier, but the Ninja Tabi are far better defensively, and it made Evasion and Nimbleness in the Defensive Mastery Tree worth taking. Normally I'd say that the dodge chance you get just from Evasion and Dodge Seals is negligible, making Nimbleness almost worthless. However, if you take Ninja Tabi, it makes Nimbleness an almost must-have.
Next you get a couple BF Swords. These give you most of your attack damage. Get both of them before you turn any of them into a Bloodthirster. After, you have both BF Swords turn one of them into a Bloodthirster. At this point you have a choice: if your team isn't pushing through towers well, then get the Stinger (or any other high attack speed item) so that you can jump towers with your ultimate. The attack speed allows you to regenerate your passive blocking ability more often so that you can continue to hit it for longer. If your team is doing fine in this category, then get the second Bloodthirster first. It'll be more helpful against other champions. At this point I leave it completely up to you. You don't even need to get the Stinger at all if you don't feel you'll need it. You can always go for more attack damage with another Bloodthirster or a Black Cleaver. I tend to go for a defensive item like Banshee's Veil (if they have a lot of CC and/or magic casters) or Guardian Angel (cuz it's just awesome). Don't expect the game to last long enough to get much past the second Bloodthirster though. Most of the time I can start on the Stinger but not finish it.
Now for the bright, colorful things that sit on your profile's rune pages and make it look pretty while still being cool and giving you stats! This is pretty simple: Armor Penetration is an amazing stat for Pantheon and is a MUST for the Marks. I've never seen a reasonably good build that didn't have Armor Penetration Marks.
For Seals, I go for Dodge. The only thing that's better defensively than Dodge is flat Health, and that is only really helpful early game. Plus this Dodge adds up with the Masteries and Ninja Tabi to give you a whole bunch of dodging power.
For Glyphs I go for Cooldown Reduction. Pantheon's abilities are relatively short, but it's amazingly powerful to reduce them further. At level 18, if you combine Cooldown Reduction runes with The Brutalizer, Stinger, and the Sorcery Offensive Mastery, you end up with 31.1% Cooldown Reduction, which brings Heartseeker Strike down to 11 seconds instead of 16. If you can get the Heartseeker Strike off that often, you'll be throwing a ton of damage around. If you don't want to do Cooldown Reduction, then I recommend Magic Resist. Always try to get the Golem Buff because it helps with your mana issues and also offers you more Cooldown Reduction. Whether you get the Cooldown Glyphs or not, you can cap out the Cooldown Reduction at 40% if you also have the Stinger and Brutalizer.
Quintessences are a tough choice. I go with Mana Regen because I have so many mana problems unless I'm constantly getting the Golem Buff, which is nigh impossible early game unless you take Smite, which is a waste unless you're going to jungle for the whole first part of the game. Anyway, there are plenty of choices if you'd rather not worry about mana, like more Armor Penetration, Attack Damage, Cooldown Reduction, Dodge, etc.
I pretty much just leave this up to you. If you feel you need more escaping power, then get Ghost or Flash. If you feel CC is generally a big issue, then get Cleanse. If you want to be able to kill more people, then use Ignite and Exhaust. If you're a lower level and don't have enough Mana Regen from the Quintessences yet, then you can get Clarity. I don't really have any way to really tell you which Summoner Spells you should take because it's very dependent on your play style and what spells you're used to.
Get all three abilities right away. It doesn't necessarily matter which one you get first, but having the Spear Shot first gives you more harass ability and greater ability to last hit minions. Aegis of Zeonia is great if you're laning with someone else that has a stun or something similar because you can get kills using auto attack pretty easily because they can't get away. Heartseeker really isn't that great of a first move to take, however if you're laning with a stunner it could give you a bit more damage output against champions, but Spear Shot's cooldown is much shorter, so you can probably get two shots with that, which will add up to a bit more damage, and it's much easier to hit with it than with Heartseeker. Later in the game, though, Heartseeker Strike is your biggest, strongest weapon; it can deal obscene amounts of damage, so we max that sucker out first. At level 8, you won't be able to take Heartseeker, so we'll take a point in Aegis of Zeonia to extend that stun time a bit to make sure the stunned champion stays in the strike long enough to take more of the Heartseeker damage. After that, max out Spear Shot to gain greater damage output and to make sure those champions that are running away with low health take enough damage to finally kiss the dirt. Aegis of Zeonia will be the last ability to max out, and of course, make sure you're putting points into your ultimate whenever it's available.
I did a whole bunch of calculations to show the awesome power of this build, but at the moment I'm feeling too lazy to put them up. I'll get to it in the future, so keep checking back here if you want to see how awesome Pantheon is.
Pantheon is an amazing killer, and with Grand Skyfall he can decide to kill anyone on the map at almost any time he wants. He is quite squishy though, so don't jump into large groups, especially groups filled with lots of CC. If you're a bit careful, he can survive a lot and still have the highest kill count on your team.