I'm a Pantheon main and he's by far my best champion, I know him inside and out. Pantheon at his core is someone who is strong for the first half of the game, before he starts getting weaker in the second half of the game. In lane, you want to harass your opponent without allowing him room to retaliate. By level 5 I have 2 points in Q, one point in W, and two points in E. His all-in combo is typically Q, W, E, Q, Ignite for the kill. You should weaken him beforehand with Q's spear shots before going all in with the combo.
It's important to note that spamming your attacks does drain his mana quickly, so I like to start flask + 3 pots (2 red, 1 blue typically). It helps him sustain himself in lane, as well as still dealing out damage without running out of mana quickly. Pantheon is a solid dueling champion with great kill potential in lane, especially pre-6 on many match ups. An alternative start would be longsword + 3 pots, which would be a fine choice when you know you can evade the enemy's counterattacks, or when the matchup is heavily in your favor.
Against Riven it's vital to keep your distance. Pantheon is great at all-ins, but there are still many champions that can beat him in an all-in, and Riven is typically one of them if played right. In these situations, you want to bait out her attacks while harassing her with spear shots. Get used to feinting her into thinking you're about to throw a spear, so she uses her E (that shield dash move), and then harass her with Q after the shield dissipates. You have the advantage pre-6 if played right. Post 6 she's dangerous with her ultimate, because it strengthens all her abilities. Watch her movements carefully. If she catches you with a stun, use W to stun her back and walk away, while throwing a Q.
All build paths depend on so many things - your lane opponent, who's fed, whether or not you're behind, etc. But in general, Pantheon is built as an off-tank. This means you build damage AND tanky items on him, so he can not only deal out good damage, but take some hits as well.
For summoner spells, if it's a lane you think you can get a kill in, go for ignite/ flash. If it's a hard match up (like malphite, shen, yorick) then go for teleport. As for items I like to start with a brutalizer and sometimes a tiamat. Be careful with tiamat though, because you will end up pushing your lane due to its passive/ active, which is why I typically build it a little later in the game. I usually go mostly damage early game, then as mid game approaches I start getting tanky items (randuins and sunfire if AD, spectre's cowl if AP). Be sure to help other lanes with Pantheon's ultimate, as it's great for surprise ganks. Late game his kill potential isn't as strong as other assassins, but he can still solid damage and be semi-tanky in the process. I wouldn't use Pantheon as an initiator to start team fights, because he gets focused and dies far too quickly. Try protecting your ADC with him, unless you catch someone with a W and your team can add follow-up CC to secure the kill.
There's so much more to write as I've only scratched the surface! I suggest watching Youtube guides of high level players to see how they handle him, as well as using the in-game spectate feature to observe their movements, as well as build paths. I first learned the game as Pantheon, so keep practicing and you will understand his strengths, weaknesses, his limits, and his potential. Best of luck to you!
