Here we go Pantheon players, let me start off by saying hi to you all. Well, I'm a big fan of Pantheon and believe me, I've tried all sorts of build in MobaFire for Pantheon and non of them seem to have a right answer to me. Either Pantheon has mana problems early to late game, or lack of damage early to late game, or not beefy/tanky enough late game or not enough gold/money to buy those expensive BF Sword and upgrade them. Well my friends, my build here is to make you go all the way to the end of the match being able to compete and dominate other champions despite Pantheon's downside in all category.

1). The Manamune. The Manamune is great for Pantheon because Pantheon is a AD caster with very poor mana, which cause so many people build down the utility masteries for mana regen and sacrifice the defense masteries that keeps Pantheon alive in a fights. Since Pantheon's spell has been buff in spell damage, Cooldown Reduction, and Mana Cost, it is a good time to take advantage of those buff and spam it early game so you can deal more damage to your opponents later on the game due to Manamune UNIQUE PASSIVE that converts 2% of your mana into attack damage. Basically, the more you spam your spells, the more attack damage you'll get.
*****ONLY 2110 GOLD

2). Mercury Treads or Ninja Tabi. All my boots are all for defensive purposes. My original build is Mercury's Treads. The only time I build Ninja Tabis is when their whole team is filled with AD/DPS Champions and with little CC on their team.
*****Mecury's Tread - 1200 ***** Ninja Tabi - 500

3). Phage-Frozen Mallet. The biggest reason to build this item is to increase Pantheon in all his most needed categories. For health and for damage. Another important reason to build this item is that Pantheon's full speed is only at 400, which why you'll need Phage or Frozen Mallet's UNIQUE PASSIVE to slow them down for you to kill.
*****This is my only expensive build at 3250

4). Atma's Impaler. This is the item that will change Pantheon forever in the game. This item is a nice combo with Frozen Mallet because of it's UNIQUE PASSIVE to convert 2% of your health into attack damage, by the time you get this far, Pantheon's attack should be way over 200. It also makes Pantheon even more tankier with the additional 45 armor and more deadly with a 18% chance of landing a lethal critical strike.
*****Another ridiculous cheap item for only 2350

5). Last Whisper/Warmog's Armor. For these 2 items, I have the option to build depending on how their how their team works. If their team is building defensively, I go with the Last Whisper to deal a ton more damage to them since the UNIQUE PASSIVE is 40% armor penetration, which means you will drain their life down very fast despite having a lot of armor. If their team is going heavy damage, I like to go Warmog's Armor because this item is another great combo with the Atma's Impaler since this item gives you a bunch of health and even more when you kill creeps and such.
*****Last Whisper, another cheap item only at 2290 *****Warmog's Armor , is the 2nd most expensive item for my build at 3000

6). Last build is for heavy Magic Resistance. Force of Nature or Banshee Veil. I build the FoN most of the time just for the movement speed and it's 76 magic resistance for late game so I can have that slight chance to get away and chase down an low champion and take more punishment from AP caster from before. For Banshee Veil, I only build this item if the whole side of the other team has a bunch of CC like stun, snare, etc. and it also gives me 375 health and 375 mana which stacks onto my Manamune and Atma's Impaler UNIQUE PASSIVE abilities.
*****Force of Nature at low price 2610 *****Banshee Veil for 2715