I got pantheon a long time ago. Back then his ult was global and fights were easier to get into. Since the range reduction patch pantheon's ult has been left in a mediocre state. However he is still a beast champ.

I take Greater marks of desolaion to shred those tanks to pieces.
Greater marks of Alacrity help in farming and after bursting your moves get the kills.

Taking Greater seals of Focus allow for easier chasing with W and more poking with Q
Greater seals of Might give you some extra damage output in the later game

I take Greater glyphs of Strength to increase damage as a whole and allow for serious damage early game

Finally I grab Greater Quintessences of Strength to get some more damage in the mix

With my masteries I go with a 21/9/0 build.

I make sure to get all of the damage and armor penetration that I can along with the life steal.

In the defense tree I get armor and health to maximise sustainability

I usually grab Teleport and Flash for my summoner spells.

Although Pantheon's ult is like Teleport and the cooldown is rather short I find that the shortened range on his ult can sometimes keep him from crucial fights and early game you can confuse the enemy by switching lanes.

Flash is always a good summoner spell to get on any champion. The ability to go over walls when running away or dodging powerfull spells is always useful.

I start the game grabbing all three spells taking Spear Throw first.
I max Spear throw first for damage output and insane poking ability.
Since Aegis of Zeonia dosent scale with attack damage and the duration of the stun dosent increase I take just the one level early and max it last.
HeartSeeker strike is your big team damage dealer in team fights. I max it second.
And as with most champions I get Grand Skyfall at lvls 6, 11, and 18

Start the game off with a dorans blade. If you can get a partner with a stun or slow it can make it easier to get first blood. Spam your Q to poke and only use your other abilites to go for the kill. Grab your boots and a second dorans blade and continue to harass. A common problem I see with Pantheon players is those who grab mana items. Pantheon dosen't need any mana items, they take up space and cost money that could possibly used for other items. Pantheon only requires mana when people use Aegis of Zeonia and HeartSeeker Strike when it is not necessary to use them. If you are in need of mana however grab a few potions or ask your jungler if you could have blue buff. Blue buff on pantheon is a *****, the nearly infinie amount of mana and the increased cooldown reduction make him an amazing chaser.

Mid game is when pantheon can really start to shine. Around this time you should have your ult boots, and, if you farmed well, a bloodthirster. This is when you should start ganking. The best way to use your ult, rather than ulting in a group of enemies, is to aim behind them. Whenever pantheon ults people panic and tend to run away, ulting behind them not only cuts off their escape but if they do run back you land on top of them. When your ult lands quickly activate Aegis of Zeonia and lead into HeartSeeker strike. If possible land a few auto attacks and end their lives with Spear Throw. Other than ganking dont fall behind on your farm and make sure to guard those towers.

Late game pantheon can either be an unstoppable force, or an underpar AD carry. If fed pantheon can have enough health to never die, and enough damage to take out the tankiest of champions. However if you have little kills, or fed, then hes not very usefull in large team fights. If your fed feel free to start the fights by ulting behind the enemy team and allowing them to focus you while your team picks them off. If your underfed wait for another teamate to initiate the fight and ult in after the fight starts to heat up.

Pantheon is a powerful champ that isnt hard to be good, but takes skill to be a master.

As this is my favorite pantheon story i have to tell it.
Our team was losing there was a team fight going on in mid and the team began to retreat. Having not been in the fight yet, i Ulted in behind the enemy's advance and they quickly turned on me. Seeing this my team came back and although I died we got the ace. My team then proceded to take out the 2 mid turrets and the inhib and nearly taking out the 2 nexus turrets. When all seemed lost as the enemy team was in our base, our Twitch back doored the base and took out both nexus turrets and killed the nexus.

If this isn't reason enough for pantheon to be an amazing champ and tanky assassin I don't know what could say otherwise. Pantheon is an incredible champ and I would recommend him to anyone.

One thing I have to say though. And i can't say it enough.
DON'T BUY MANA ITEMS FOR PANTHEON!!!!!!!!!!