Hey, I am Lamenough an average player.
I use these builds for Blind and Draft pick. I would recommend using another build for ranked because I don't have that much experience with ranked games.
Pantheon was the 3rd champion I bought from IP and I got him like 5 months now and he is absolutely amazing in my oppinion. The Spear Shot (Q) for poke the stun, 100% critical chance when enemy below 15% health and that passive, Blocking a tower hit when tower diving early game is really useful!

Pros:

Can be build tank and bruiser
Can be used in Jungle, Top, ADC and mid lane!
High damage early and late game if build well.
Is great at 1v1 because of the burst damage / combo
1 second stun can be unlocked at lvl 1.
100% critical chance when enemy is below 15% hp (helps alot in 1v1 and tanky champs)
after 4 basic attacks Pantheon grants Aegis Protection to block attacks.

Cons:

Block Doesn't block all attacks only basic attacks, tower hits and some spells.
Can be a pain in the *** to play when losing the lane.
Needs early game farm to get Phage for sustain and Brutalizer for damage.
Can be easily shut down because of no escaping mechanism.
No defensive skills

Pantheon, the Spartan. The champion who has the same shield as all the soldiers in the movie 300.
Lets just talk about Pantheon and these are notes you should know:

Passive needs 4 basic attacks to get the protection
It only blocks 1 attack.
Spear Shot (Q) is your main damage dealing skill
Grand Skyfall (R) damage is 100% in the middle and drops to 50% when the enemy is at the outside.
Grand Skyfall (R) will slow enemies you hit for a short time
Never think Grand Skyfall (R) will do maximum damage.
Spear shot has a 4 second Cooldown.
Skill order: Grand Skyfall > Spear Shot > Heartseeker Strike > Aegis of Zenonia
It is Optional to max. heartseeker Strike first but I reccomend Spear Shot.

Combo without Grand Skyfall (R)
1st Spearshot (Q)
2nd Aegis of Zenonia (W)
3rd Heartseeker strike (E)
and than end it with 1 more Spearshot (Q)

Combo with Grand Skyfall (R)
1st Grand Skyfall (R)
2nd Aegis of Zenonia (W)
3rd Heartseeker Strike (E)
4th Spear Shot (Q)

Jungling combo
1st Spear Shot (Q)
2nd HeartSeeker Strike (E)
3rd Smite
and after spam basic attacks or skill shots

Pantheon is an Assasin so play him like one!
Pantheon could start a team fight by stunning the adc or apc or the fed top.
Pantheon can start the Teamfight with his Grand Skyfall (R)

I would recommend the summoner spells:
Flash: Almost a must for every champ. It can close or open the gap between you and your enemie.
Teleport: Really usefull when playing Pantheon. instead of just using your Grand Skyfall (R) on mid lane and walking back you can easily teleport back to stop the push from your lane partner.
Exhaust: You might think how does this help pantheon he has a stun!. Altough it is true sometimes the combo isnt enough to kill some one. Exhaust lowers their armor and movement wich makes them stay longer in your Heartseeker Strike (E). With exhaust Heartseeker Strike (E) is hard to miss! If your enemie is faster than you just exhaust him / her than start the combo.
Smite: Only when Jungling. this might not be smart but it works good in blind pick / draft pick because no one counter jungles: When you unlock Heartseeker Strike you will do 100% critical chance when the enemie is below 15% health. wich makes jungling faster if you use smite first and just spam basic attacks when the enemie is low on hp.
Ignite: I'm not a fan of ignite but It is Optional to use it.

I can easily explain this.
at lvl 1 and you are scared last hit by using your Spear Shot (Q)
at lvl 2 Try to poke the enemy and cancel his farm a bit so he gets scared of you.
at lvl 3 Harass more. do a short combo and make him half hp wich will help you and your jungler by securing the kill.
at lvl 4 Harass and get the kill!
at lvl 5 becarefull some enemie champs can now be dangerous and you lost your early game advantage of free damage.
at lvl 6 gank mid with your ult or recall get items ult on your lane partner get the kill in a burst and than farm alot for more gold. (watch out for ganks)
after lvl 7 just farm, outplay and gank. There is not much to do except for getting as much gold so you get assasin like late game maybe early game or even FED!

When your farm gets canceled it is most likely because you are scared or you fed the enemy. ask the jungler to gank when you poked your enemie so hard so he is around half hp. Try to get the more farm than him / her and try to get on the same lvl.

Q&A / notes

Q: Why should you imidiantly use Aegis of Zenonia (W) after Grand Skyfall (R) ?
A: when you are in the air, use your Aegis of Zenonia (W). When you fall down you will instantly leap towards the enemie and stun him so you can start your combo. This will stun him in your Grand Skyfall (R) . The enemie will take damage from your Grand Skyfall (R) and Aegis of Zenonia (W).

Thursday 24 - 01 - 2013, added Chapters 1, 2, 3, 4, 5, 6 and 7.