I like to play Pantheon because he is a great assasin and good to last hit with. I think all you that like AD assasins should play him or at least test him. Note that my English isn't very good and that im not a real LoL pro.

I take critical dmg quintessence's because heartseeker strike grants 100% critical chance to targets under 15% health. Seals of vitality is important in order of Pantheons low base health.
Armor pen marks is obvious in order that my build is AD. Then we have the glyphs, on them i take mana regen because Pantheon is a real AD spellcaster.

On masteries i take 24/6/0 to deal highest dmg. I also take some defence masteries because Pantheon is a real glasscanon. In order of that i also have lifesteal.

Now to the items. I always start with boots of speed and 3 hp pots on all AD's i play, pots to stay longer in lane and shoes to make sure i can escape but my target can't. On my first trip home i upgrade my boots to berserkers greaves. After that i start building my bloodthirster.
I often start with the vampiric scepter but if i have money enough i start with bf sword. Then i start with my phantomdancer to get even more speed and some critical chance. When i have my phantom dancer infinity edge is a must because it increases critical damage. Then i unbelievably take a tiamat to regen more mana and health and because its damage, and it's splash effect is real good because it makes me able to kill more minions and get more money. last but not least i take the black cleaver because of dmg attackspeed armor pen.

I always start with spear shot so i can take first blood with it. And with that one i can easy stack up to my block wich makes it all much easier. Then in lvl 2 i take aegis of zeonia because it refreshes my block and stuns the enemy. First at lvl 4 i take heartseeker strike. I max spearshot in lvl 9 because i think its the best attack. What it comes about aegis of zeonia i take 1 point early but max it last, in lvl 18. The only use i see of it is stun and refresh of block.

I take flash and exhaust as summoner spells because both of them are good both while hunting and while escaping. The most people prefer ignite instead of exhaust but with exhaust you get more time to deal dmg.

Pantheons ultimate is real hard hittng with so just before i land i use aegis of zeonia. His ulti is real good to gank with. A little hint is that its easier to use on a jungler because in jungle its harder to see the circle. Thats all i have to say thanks for reading my Pantheon guide!