Hello, I'm a Platinum I player and I mained the old Pantheon for like 3 seasons. Now that he finally got reworked I wanted to do a guide on how to play him. The guide is still in work so hope you understand that. I hope you learn something from it and it is helpfull for you guys ^.^.

Pros:

Strong Laning Phase
Point and Click Stun
Range Harass
Execute Targets with Q at long distance
Blocks lots of damage with E
Semi-global ultimate

Cons:

The Q is hard to land on long distance
Ultimate hard to position correctly
Hard in teamfights
Falls off if not played correctly

Mortal Will (Passive): Pantheon gains a stack when he lands a basic attack or casts an ability, up to a maximum of 5. At 5 stacks, Pantheon's next basic ability consume the stacks to gain additional effects.

Pantheon starts the game with full stacks. His new passive is really good and makes the reworked Pantheon a better choise than the old Pantheon. Stacks management is a thing you must learn. Against champions in lane you will usually prefer to stun them with 4 stacks to have your Q empowered ready to use.

Comet Spear (Q):

FIRST CAST: Pantheon Channeling iconwinds up for up to 4 seconds, increasing Comet Spear's range while being Slow icon slowed by 10%.

SECOND CAST: Pantheon Projectile hurls his spear, dealing Attack damage physical damage to all enemies hit, reduced by 50% to enemies beyond the first, and increased against enemies below 25% of their maximum health.

PHYSICAL DAMAGE:»
75 / 115 / 155 / 195 / 235 (+ 100% bonus AD)
Releasing the ability within 0.5 seconds causes Pantheon to instead thrust his spear, dealing physical damage to all enemies hit and Cooldown reduction icon refunding 50% of Comet Spear's cooldown.

THRUST DAMAGE:
75 / 115 / 155 / 195 / 235 (+ 100% bonus AD)
If Pantheon is interrupted during the charge, or the charge completes without reactivation, Comet Spear is cancelled and the ability is put on full cooldown but refunds Mana icon half the mana cost.

Mortal Will: Deals an additional 20 − 241 (based on level) (+ 100% bonus AD) bonus physical damage and Slow icon slows the target for 2.5 seconds.

SLOW:
20 / 25 / 30 / 35 / 40%

Now your Q have 2 parts, the throw and the tap. In long fights against a champion you will want to use your tap (it refunds half of your cooldown which is so good) if the champion is melee. If not your thrown is your friend. Try to use it when there are no minions in front of your enemy, in that way it will deal the maximum damage. In close combat when the enemy is below 25% you will want to throw it in him, even if he is in your basics range, you want that execute to happen (don't do it if you are facing let's say a fizz and he have his E).

Shield Vault(W):

ACTIVE: Pantheon dashes to the target enemy unit, dealing them physical damage and Stun icon stunning them for 1 second.

PHYSICAL DAMAGE:
60 / 80 / 100 / 120 / 140 (+ 100% AP)

Mortal Will: Pantheon's next 3 basic attacks deal 50% damage and occur in the span of one attack, dealing a total of 150% damage. The flurry of attacks all behave like standard basic attacks.

This W is basically the old W with a empowered version in his kit. Remember your empowered version applies on hit effects, like the Blade of the Ruined King one. You can still flash+w+ignite+q into enemy to finish them but you are way slower than before so watch out.

Aegis Assault (E):

ACTIVE: Pantheon braces his shield for 1.5 seconds in the target direction while striking in a cone, during which he is Slow icon slowed while not moving forward, up to 50% while moving backwards. Aegis Assault cannot be interrupted by crowd control.

For the duration, Pantheon is Cosmic Radiance invulnerable against enemies and Turret icon turrets in the target direction that attack him, and deals 「 up to 100% AD over the duration 」to all enemies struck, reduced by 50% against Minion icon minions, and by 15% against Monster icon non-epic monsters.

At the end of the duration or upon reactivation after 0.3 seconds, Pantheon slams with his shield, dealing physical damage.

SLAM DAMAGE:
55 / 105 / 155 / 205 / 255 (+ 150% bonus AD)

Mortal Will: Aegis Assault's duration is increased to 2.5 seconds, dealing「 up to 166.6% AD physical damage over the duration 」.

Your E is the newest coolest upgrade to your old spells. You become practically immune to all damage while facing the source. You can dodge negate Karthus Requiem, Pyke Death from Below, Darius Hemorrhage and many more with it. Remember that it only blocks the damage for you and not your allies.

Grand Starfall (R):

ACTIVE: Pantheon Channeling iconchannels for 2 seconds to leap high into the air, then marks and reveals his destination before crashing down along the target area 2.4 seconds later. He remains vanished until the landing is complete.

The landing deals Magic damage magic damage, decreased by up to 50% at the edges of the area.

MAGIC DAMAGE:»
300 / 500 / 700 (+ 100% AP)
Grand Starfall instantly readies Mortal Will.

Grand Starfall's Cooldown reduction icon cooldown is reduced to 30 seconds if the channel is interrupted or canceled.

Remember that your final position will be where you point that R. So keep in mind that you will need to put your ultimate far behind your enemies to really do something with it. While you are in the fall down you are immune to anything.

You can sometimes take E at lvl 2 instead of W. If the enemy laner pokes you too hard feel free to take it.

Conqueror is one of the best if not the best mastery to take on Pantheon, it gives you AD, true damage and healing which you all need. Triumph helps you with survivability. Tenacity is always good on the immobile champions and Coup de Grace helps with your Q execute.
Inspiration in my opinion is the second best, the Biscuits helps you with sustain and Time Warp Tonic is really in hand in engages.
2 adaptives and 1 armor/magic resist is what you need to take.

If you want have a squishy enemy comp you can take the Dark Harvest to kill their back line in 2 seconds.

There down is a playlist made by me some days ago for the new reworked Pantheon, if you want some ambience which fits the champion theme you should try it out :D
https://youtu.be/-acSt3pYWpU