Before Reading This Guide let it be known, I like to be comprehensive in my explanations so they may be longer then you are used to, with that said Enjoy the guide.
Hello and welcome to my guide to The Artisan of War, This Guide will cover the way I play solo top . Things you should realize going into Pantheon top is that he is:
1. A counter pick to many tops.
2. A bully in lane.
3. A mana hungry champ is you don't play him right.
4. He is an AD CASTER, NOT an AD DPS.
Knowing these 4 things before going into a game will improve your playing before even looking at runes, masteries or items.

Throughout the guide I use certain words to rate runes, masteries, items, etc. Here's an overview of what they mean in association with a 10 start rating system.
Very Bad - 1 star
Just No - 2 Star
Not Viable - 3 Star
Not recommended - 4 Star
Decent - 5 Star
Recommended - 6 Star
Quite Viable - 7 Star
Good - 8 Star
Very Viable - 9 Star
Amazing - 10 Star
Now you know.

Pantheon's Skill Leveling goes as follows, R>Q>E>W.
We'll Start with Pantheon's Q, His Spear Shot, its his main harass and most spammable spell. Its where most of your damage is going to be coming from in lane and most of mid game so use it wisely. In lane you'll want to lay down harass often but not extensively as to not drain your mana pool, so be wise with this spell. Finish Leveling it by level 9.

Pantheon's W, his Aegis of Zeonia, is his 1 point wonder skill, all you need is one point in this and you're good. Its a 1 Second stun which allows Pantheon to land his full combo (going to be gone over later in this segment) but is definitely not what you want to be investing your points in early, finish leveling it by level 18.

Pantheon's E, his Heartseeker Strike, is his primary damage late game as its scaling and uses really only matter when you get a good amount of AD, it is important early and mid however how its damage in lane although not huge is noticeable. Its main usage throughout the early and mid is its passive which says that Pantheon will automatically crit a champion, monster, minion once they drop below 15% health, finish leveling it by level 13.

Pantheon's R, his Grand Skyfall (or Mandrop), is his Ultimate which allows Pantheon to jump to a place on the map within 5500 Units way (approx. half the map). Use this ability in teamfights to cause mass confusion as it will deal massive damage to opponents who stand in the middle of it when pantheon lands, be weary however as it has a long windup time and is easily dodged, as with most ultimates you level it up at levels 6, 11 and 16.

Pantheon's full combo goes as follows Q,W,E,Q. You use your Q to initiate a skirmish, use your W to stun them, which will give you enough time to land your E, then once they start running or start to fight you your Q should be up in time to throw it once more. Landing this combo will get the most damage in the shortest time.

My Item Path builds for this, I do damage to everything and I care not about anything. With The Brutalizer and Last Whisper in your build you will kill just about anything in your path with no problems at all.
Boots are really a mixed bag on Panth, none really work perfectly for him but typically you'll be either getting Ninja Tabi, Mercury's Treads or Ionian Boots of Lucidity. Ninja Tabi is for when you aren't doing well in lane and are harassed, Mercury's Treads are your regular boots so that you wont get pinned down easily and Ionian Boots of Lucidity are for more damage as you are an ad caster.

After getting your damage core you start on survivability through [frozen mallet], Atma's Impaler and Banshee's Veil. These 3 items allow you to survive long enough to get your burst off and stick to any target. After finishing your tankiness your build truly just opens up to whatever you want your last item to be, I find Infinity Edge to be a good buy as you have your auto crit from your Heartseeker Strike E.

I feel I need to explain myself here, rushing Last Whisper is NOT super conventional, HOWEVER it is what you will need if you are doing what you need to in lane, wrecking it. So please, don't just look at this on paper, try it for yourself before you bash it. As the saying goes, don't knock it 'till you try it.

Other Honourable Mentions are:
Bloodthirster: Good for Damage and Sustain, but you typically will either die or not get the chance to farm it up. Quite Viable.
Youmuu's Ghostblade: This is a tricky item to assess as it truly seems like an amazing item on Panth, but paper and field are 2 different things and this just does not work as well as it would seem. Recommended.
Trinity Force: Its ok, it gives him a little bit of everything which really can help, but doesnt synergize with Frozne Mallet Very well. Quite Viable.
Hexdrinker: Only if the other team has something like a karthus, although it does synergize decently with Banshee's Veil Banshee's. Decent.
Wriggle's Lantern: Wriggle's is ok on Pantheon but honestly, you don't need it. His Laning presence is strong as is and for sustain pots can tide you over. Decent.
Warmog's Armor: A good substitute for Frozen mallet, good if you decide to go Trinty Force. Recommended.
Randuin's Omen: Quite a good item, good synergy with atma's and frozen mallet and it can shut down the carry very easily. Good/Very Viable.
Chalice of Harmony: Yeesh, why do people get this on Pantheon, you don't need the Magic Resist (In this form) or the mana regen this thing gives. Only get it if you want to throw out your Spear Shot q's ALOT. Not Viable.
Quicksilver Sash: QSS is good if you find yourself getting locked down to the point where Merc's just don't matter. Recommended/Quite Viable.
Keep this in mind for whatever item choice you may make, you need to be able to be dealing steady damage while taking hits, if you accomplish this you're doing your job.
Force of Nature: Still a very respectable item, but I find Banshee's Veil to be a bit better as it stacks more daage onto atma's. It's really all preference though. Quite Viable.

Their are 3 different rune pages I use for Pantheon, the one I use the most is the one mentioned above, The greater mark of desolation ArPen is there to give 20 ArPen once you get your The Brutalizer.

The extra Greater Mark of Attack Damage ad is so that your harass is more powerful.
The Greater Mark of Armor armor is so you can survive harass.
The Greater Glyph of Cooldown Reduction Cdr is so that you can easily harass as your main form of damage (your Spear Shot q) will be off cd more often.
Quints are really whatever you want them to be, you could go Greater Quintessence of Attack Damage AD, greater quintessence of desolation ArPen, greater quintessence of focus] ]CDR (although I wouldn't recommended it) however, I feel that [[greater quintessence of swiftness MS stand above these other choices for the sole fact that you want to be able to run in and out to harass your opponents at top, the same principle applies for people like Warwick, you want to run in, Hungering Strike Q then run out.

Other choices for runes are:4
Greater Mark of Critical Damage Crit Damage Marks. Give your auto crit more power. Not recommended
Greater Seal of Critical Damage Crit Damage Seals. Give your auto crit more power. Not recommended
greater seal of vitality Health Per Level Seals. Don't protect you from harass early. Decent, stack with Atma's.
Greater Seal of Scaling Mana Regeneration Scaling Mana Regen Seals. Good for Spamming spells. Recommended.
Greater Glyph of Armor Armor Glyphs. Won't get harassed easily early in conjunction with armor seals. Decent.
Greater Glyph of Magic Resist Flat MR Glyphs. Take if you're afraid of AP Bursters Early Game, very viable choice. Good.
[greater glyph of shielding]] Scaling MR Glyphs. Take if you're afraid of AP Bursters Late game, Very viable choice. Good.
Greater Glyph of Scaling Mana Regeneration scaling Mana Regen Glyphs. Good For Spamming Spells. Recommended.
Greater Quintessence of Health Flat Health Quints. Take them to be tankier early game. Decent, Their are better choices. Decent.
Greater Quintessence of Armor Take them to take basically no harass in conjunction with Seals and Glyphs. Decent.
greater quintessence of desolation Take them to shred through possible Cloth Armor Cloth Armor and [Wriggle's Lantern] Wriggles. Very Viable.
Greater Quintessence of Critical Damage Take them to boost your auto crit. Not recommended.
Greater Quintessence of Attack Damage Take them for better harass early. Quite Viable.
Greater Quintessence of Cooldown Reduction Decent for spamming spells. Decent.

In Summation:
The Ideal setup is one where you can Harass without taking to much damage. The ways to get this are through Armor, AD, ArPen, CDR/Mana Regen.

Masteries are pretty straight forward, You take everything ad related in offense including Sorcery cdr so you can harass simpler, this in conjunction with your Greater Glyph of Cooldown Reduction cdr runes will allow you to harass alot easier.
The Main points in offense that you want are Lethality Crit Dmg, and Weapon Expertise ArPen so that you can deal the most damage in lane.
For the defense tree its really up to preference, you can take 8 in Utility to get 3 points in Expanded Mind expanded mind to get more mana, but really other than that and Swiftness the movement speed, so defense is really the better choice.

In the end you'll really want to take most of all the ad in offense, and spending any leftovers in utility or defense, both will help you in lane but I feel that defense has a slight edge over utility.

Summoner Spells for Pantheon really are trick to recommend, some people like Exhaust exhasut, some people like Ignite ignite, some people like Ghost ghost over Flash flash some people swear by Flash flash, its really all preference but, let me explain my reasoning for taking [flash] [ignite].

The combo of Flash flash and Ignite ignite is the most aggressive combo in the game, and at top lane your job is to harass and shut down the opponent, in other words be aggressive. Flash flash and Ignite ignite give the killing power you need because sometimes that auto crit just wont be enough.

That being said their are plenty of suitable replacements, and I'll list them:
Ghost Ghost: Not quite as aggressive as you'd like on pantheon n my opinion. You already have plenty of movespeed anyways. Good.
Exhaust Exhaust: Exhaust is the spell you would take if you plan on doing alot od dueling instead of bursting although the slow can allow you to catch up to get that last hit in most of the time. Good.
Heal Heal: Heal is kinda meh on Pantheon all around the board, he doesn't need the sustain, for the most part if your opponents smart they can get away from you before you land the finishing blow in which case you would rather have ignite or exhaust, I feel the above 2 spells are of much greater use. Decent.
Teleport Teleport: Its, ok you already have a port if you need it and typically if you're going back you've already forced the other person out as well. Take when you're first starting out and don't feel all that confident. Not recommended.
Cleanse Cleanse: Now this one seems almost completely worthless to me, but I do see people taking it from time to time so I guess I'll go over it. It really just doesn't have a spot on Pantheon Pantheon's choices at all, [Quicksilver sash] QSS is a better choice and its cheap with good MR not to mention it gets rid of more things. Just no.
Clarity Clarity: Now this one is just plan stupid, clarity's a bad spell to being with but to put it on someone who (although has some) doesn't have many mana problems in the first place is honestly just stupid. Not Viable.
Clairvoyance Clairvoyance: Clairvoyance really is a strange pick and it only serves one purpose, finding people to Grand Skyfall Mandrop onto. I find it to be completely useless and a terrible choice for a summoner spell. Just No.

I have said this before and cannot stress enough, icon=Pantheon size=32 PANTHEON IS A COUNTER PICK, he is not to be chosen as a solo top first, you see what the other team has and you use as a Check or Counter. People like, Nasus Nasus, (sometimes) Trynd, Irelia, Volibear, Yorick, Trundle and Riven are examples of who Pantheon counters or checks, all of them are the same playstyle, poke with Q a few times while farming, hit level 4 and they should be low enough that you will kill them. Give the enemy no time to think at all in lane and constantly harass them until you can go in for the kill.

Now, since icon=Pantheon size=32 Pantheon is a counter pick he does lose to some people, from my experiences people that pantheon has trouble are people like Ryze, because he has safer harass and people like olaf who can shrug off the harass, watch out for people that fit this criteria.

To conclude, Pantheon is one of the strongest counter picks in the game, his match work for him very well and he can crush his lane if played properly.

Thanks for reading my guide, feel free to leave feedback if you wan.
And remember summoners, watch for circles forming above you.