There's no One True Build, but here's some guidelines, and the way I generally go.
Pantheon is an AD caster; the majority of his damage is in his abilities. This means that raw AD is more valuable than **** near anything else, and it means that attack speed and crit are fairly useless in comparison to raw AD. Builds going with Zerker's or Black Cleaver or Phantom Dancer are bad builds; you shouldn't be autoattacking enough for the attack speed to be worth it. That means that when you want to build damage, you want to build AD.
Myself, I start boots/pots almost all the time. Panth needs the mobility to harass with his spear and to avoid harassment himself.
If the game is decently for me, I go straight for a Brutalizer next. Brut is perfect for Pantheon: AD, ArP, and CDR. Everything you need. This is my first choice that gives you a surprisingly large amount of early game damage due to Panth's huge AD ratios. Hexdrinker is a very close second, though. The same AD as the Brutalizer, but with a lot more survivability against AP. I get this against AP tops(unless I'm stomping them), or when the mid is somebody like Cass and I know I'm gonna need the MRes.
As a note, the Brutalizer builds into a Ghostblade; the Brutalizer is excellent for you while the ghostblade is a waste of gold. The upgrade costs as much as the Brutalizer again, and the stat upgrade is 5AD, 5% CDR, and 5 ArP. The active is only really useful as a chase ability for you, since you don't benefit much from attack speed. So you're paying 1300+ more gold for minimal stat gains. The Hexdrinker, on the other hand, builds into the awesome Maw of Malmortius, which is a great offensive/defensive hybrid item.
For boots, I go with Mercury's Treads or Ninja Tabi. Occasionally, I will take the CDR boots, but I generally like the defensive boost from either of the others. I choose the former against teams where I need the MRes or the Tenacity, and the latter otherwise.
At this point, it's all situational. If the game is going well, a Bloodthirster for huge AD. If it's going badly and I can't be sure of keeping my stacks up, I'll go more defensively. Maw, Fratma, Atmog's, etc. For pure defense, Guardian Angel is a solid pick.
Whatever you do, just remember that you're a caster. Your spear has a 1.4 ratio on AD. Your Heartseeker, if you land the whole thing, has a 3.6 AD ratio and it's an AOE. You want AD way more than any other offensive stat. You do not want to be one of those Pantheons building attack speed and crit and autoattacking constantly, because that means you're being a bad Pantheon.