Ok. So here is the way I normally build pantheon. I build him for survivability based on attack speed and crit power. It may not make you carry your team but it will make sure that you will survive to be able to finish what your teammates cant! So here we go!

RUNE TIME

For my quints i go with
Greater Quintessence of Desolation I use the greater quintessence of desolation, so many times you can get blocked from a kill simply by a little bit of armor, so we solve the problem in the rune page!
The reason i chose these for my marks is since my build doesnt put a huge amount of focus on building attack damage its nice to have some from the rune page, also having the flat damage for the start of the game is very useful
Main reason for this choice.... survivability, this build uses next to no armor and survives based on lifesteal, so early game some armor is a nice way to make sure you can survive.
Here i deviate from a few of the other pantheon guides, I prefer having higher mr at the beginning of the game because there are many items that i like that can give me more if needed, however the per level runes would not be a bad choice either.

I realise that my items are diferent from a lot of builds which arent wrong but they arent m style so here is my explanation:
: I normally buy 1 of these to start then another after my tier one boots, they are a great stackable and give good damage and lifesteal however after wriggles or zeal i normally sell them as their usefulness begins to wear off.
: Boots are always suited for whichever game you are in, but these are my standby choice because with attack speed comes more lifesteal (and if you pick a wits end more mr)
Madreds Razors : i get these as the first part of wriggles because we already have the lifesteal we need from the dorans blades which will be replaced once we get wriggles
: Good item for armor, lifesteal, and damage. Also the freeward can save you from ganks when placed correctly.
: Attack speed, crit chance, and movement speed. good item plus this gets your shield back up quicker!
: More attack damage! not to mention more crit chance and crit damage! all around good item!
: incresed speed crit chance and movement speed!
: Ima level with you i dont have a specific end item, at the point in the game where you could get it it really just depends on what you want, id suggest something with attack speed though (if you havent noticed yet i like attack speed)

Optional/Also good items

: good attack speed and a great item for helping you take down those annoying ap carries!
: A good item for attack speed and health, plus its always fun to see lightning bounce around on the enemy champions!
: Help your friends! If you wanna be a nice helpful player pick up this item if the other team has a lot of armor!

There are many more useful items that can be put on pantheon, I'll add more later but these are the main ones that i normally get.

Aegis Shield This is, of course, your passive. Its a nice little shield that blocks either the next turret hit, or the next basic attack. Its a nice little helper when needing to dive just a tiny bit for a kill. Its also nice for stopping ashes super crit when she waits to hit you! However it doesnt block abilities so your shield is useful but its not gonna save you if you play stupid.
This is what i max first. Its incredibly useful for poking early game, as well as criting a champ as they flee. It also scales well of of attack damage. Its a great pking tool but keep in mind that you dont have a ton of mana so use it wisely at least at the start of the game.
I max this one last. This is a very useful stun, which can be used to get a kill, save a teammate, or stop an enemy in tower range. It doesnt work on minions so it cant be used to flee, also since you are sorta squishy you probably wanna be careful when you use it.
I max this one second. Its passive is incredibly useful and awesome. Once an enemy champ is below 15% health you are garunteed to crit! YAY this makes it way easier to duke it out with an enemy champ since your main goal is just 15% while theirs is 0. Also this is a good skill to thin out minion wave and to deal a good bit of damage to a stunned champion or to a few charging champions(although you should try not to allow yourself to be charged by many champs).
This is your ult. Now it may be tempting to use this to initiate but DO NOT DO THIS. trust me the giant circle is a dead give away and if you jump into a group of enemy champs without backup being forthecoming you will get owned you can only stun one, leaving the others to pounce on you. You ult is useful for ganking (when backed up), getting into a team fight (after it has started), catching fleeing champions, and finally going behind a tower to finish off a champ (lots of fun!).

Ignite, the backbone of so many summoners kills. I enjoy this spell a lot because of the true damage and the continued health decay. Its very useful for finishing off a fleeing champ or for wearing one down that has a heal (although most people would say this is a waste, which it probably is_)

Flash, another classic choice. Its great for escaping and for jumping in to get the last hit for that crit (little rhyme there!). With practice this can get you over walls and let you escape easily into jungle!

Other Good Ones

Ghost, great for all around getting places, running away, and of course chasing enemy.

Nice for getting you back to lane when you have to go b a little early also helps you get where you need to be to support team mates.

Good one for slowing down the enemy champ so you can get in range for your spear or stun, also good for escapes and helping your team.

Well there it is, my first guide. Please dont downvote without trying it, or at least an explanation of why. As I know I'm not the best player in the world I would love to here suggestions on how to better this build. Changes will come with when I find something new or feel like adding more explanations! GL HF