This is a pantheon build directly focusing on attack speed, critical damage & chance and pure dps. However i will cover situational items.

Many guide feature pantheon using spear to harass and leveling up heartseeker as quick as possible, however, this leads to reduced harassing ability. Your main goal is to use the spear to harass, and to use as an entry attack as well before aegis. Therefor a good way of using it would simply be to level them up consecutively after each other. Use spears to catch low health enemies, and heartseeker to do massive damage after using aegis. Remember to use your ultimate as a tool to backdoor gank, block a escape route or simply to finish someone who got away, position it around 600-1000 range infront of your target.

The marks of desolation are to increase your early game effectiveness, the armor pen will allow the damage the spear toss will do and will ultimately allow you to do massive amounts of damage early game.

The glyphs and seals are to increase the damage done by crits and how often you can do so.

The quintessenses are to increase the armor pen and critical chance.

Get a Quicksilver sash if you are often cc spammed to death, it'll help your survivability.
Buying a Thornmail against other auto attackers will help your survivability and your ability to combat them.
Buy ninja tabi or mercury treads if you feel your a bit to squishy early game.

The best lane partners for pantheon are:
Amumu - ranged stun, allows extended period of dps, ultimate allows pantheon to free hit enemies.
Blitzcrank - allows you to catch any runners, also silences.
Renekton - Can dash in with and stun the target. Can also lane for extended periods of time.
ChoGath - Silence, and rupture slow, must i say more?
Alistar - Stun, Heal, Headbutt, a extremely good damage sponge, will really help you lane longer and get the kills.

Your worst enemies should be:
Ryze - Large burst damage potentially putting you out of the fight quickly.
Jax - will dodge your crits and can easily get away with his jump ability.
Akali - unable to burst her down due to her shadow bomb.
Shen - will keep saving near kills

The End.