As above has mentioned you want spear first, 1 point in HSS for passive and then your stung as it increases in duration with levels now.
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Pantheon strives as an early and early-mid game carry, but his late game is quite a struggle. Max spearshot first, then Aegis, with 1 point early (level 2 or 3) in HSS for its passive. Try to maximize your early game as much as possible by rushing a brutalizer and other +AD items. ArPen runes are also very important, since enough ArPen will give some squishies 0 effective armor at level 1, meaning your spearshot hurts a LOT. Actually, even Garen only has around 30 armor or so at level 1, so you can see how having 20 ArPen helps a lot early on.
If you make it to late game (to be honest if you're in late game you probably didn't do that well early game OR your team had literally no one to close the game out), and the enemy is stacking armor you can look into getting a Last Whisper. Since LW now affects physical abilities (read: HSS and SS), then the percent armor reduction becomes very useful.
The one thing you need to remember is that Pantheon is simply a caster, although if you wanna nit-pick he's a physical caster. That means his job is to use all his spells, then run and wait for cooldowns, just like any other caster. He's very squishy without any defensive items but his damage output can really devastate a squishy. He has no escapes either, so taking summoner cleanse isn't a bad idea, but the better option would be to never get yourself in a situation that involves you vs multiple enemies (as in, do not chase low HP squishies into a group of enemy champions; you're not Kassadin and can simply blink out when you're done).
I'm kinda tired so I'm just rambling random tips about Pantheon that I'm remembering...