While some people think AP Sion is just absurd and he's even OP, realize this is only every 8-12 seconds that he can do this nuke, and it's a total waste of his Enrage and Cannibalism. AD Sion can 1v3 late game if he's built right.
Pros
-Two second stun at level 1
-Powerful basic attack early with Enrage
-Godly ultimate with team lifesteal
-Decent backdoor if needed
-Can lifesteal nukes
Cons
-Not an excellent minion farmer early
-No escape mechanism
-Exhaust DESTROYS him
-Stuns shut him down

Optimumly, as with any carry you want tons of ArP, but with 60-90% Crit, crit damage is almost as good. For Seals I take attack speed but dodge would be helpful too. Glyphs, the attack speed isn't as great, but we want to be able to take advantage our bonus enrage damage early, but also having magic resist is nice both early and late game since Sion is very vulnerable to it with only 30 MR. For quints and marks, either crit damage or ArP will give godly damage late game.

Cryptic GazeA two second stun at level one, lets all admit it, is surely over powered, there isn't much even close to that in the game, with flash and ignite, it's easy to get right next to a sorry noob who didn't think Sion could bushwhack just got rocked. but getting as many ranks of Enrage is good too because having damage equivilant to a Pickaxe at level 2 and a BF Sword at level 7 is amazing, especially with a little attack speed, people complain of no escape mechanism on Sion, but it's much easier to ace them then port wherever you are afterward. Also, if you are any good at last hitting (should be easy hitting 100 damage at level 2) You should have an aditional 300-600 health mid to late game.
Some people argue that Death's Caress is such a great nuke with Sion's stun, but you can't get any value out of it unless it's your first focus and late game, 300 damage is one lifesteal hit, or less...
Cannibalism Is your teamfighting dream, most people think that Sion isn't a huge priority, so only one person will be focusing you, even if a few people are, you can lifesteal with this, plus your whole team gets half of what you get! This is so great in early fights too, it's easy to for a turret dive with this, 50% speed is such a boost early too. Remember, if you get focused, you didn't wait long to go in.

Grab a Vampiric Scepter at the beginning and then get some Berserker's Greaves as soon as possible. Remember, we really don't need damage since we have our Enrage for early damage, but don't forget it costs health to attack, so a vamp scepter is a nice way to counter act this early. Phantom Dancers are who you are.Let me say it again,Phantom Dancers are WHO you are. The sooner we have a phantom dancer, the sooner we can drop that squishy mage that's been harrassing you all game. As midgame/endgame starts, grab two Bloodthirsters then feed them up to full stacks as soon as possible, get another Phantom dancer and Black Cleaver for a little more speed and damage, plus some beautiful ArP. If the game lasts long enough sell your boots and get a third Phantom Dancer, then Crit away. Instead of getting a Black Cleaver, you can also get an Infinity Edge, 250% crits after already having almost 100% crit chance is simply amazing, it's important to look to see what the other team gets. Often you don't get to do exactly the same build every time.

Sion is very squishy in this build if he isn't seriously minion fed, like 500 minion kills...
An extremely great item after a Phantom Dancer is a Banshee's Veil, this item will ruin ANY nuke and it has some health and magic resist, also it solves any mana problems Sion may have. Another item for just a little health with a little damage is a Frozen Mallet, this gives a nice slow effect too.
If your team is really hurting for a tank, grab a Warmog's Armor and a Atma's Impaler, this still gives you some nice damage in addition to Enrage.
If the other team has someone with a lot of dodge, such as Jax, you can get a Sword of the Divine, the active is great with an little ArP included in the no-dodge deal.

Flash is a good way to get to your enemies early after you stun them to get the extra hit off that you need to kill them before that annoying turret decides to turn the tables. It's also a good OMG I NEED TO GET OUT spell late game.
Ignite is a great finisher for the noob who flashed away with 60 health and doesn't deserve to live. Also, ignite on spells like Transfusion, Maximum Dosage or Wish, or any healing for that matter, its good to have.
Exhaust is good for that ranged carry close to killing you that does too much damage, it's also a great chaser/getaway spell on for Sion.
Cleanse is nice for that ignite ruining your ult's lifesteal OR that stupid exhaust ruining all your damage. Spells like Counter Strike or Enchanted Crystal Arrow can totally shut down Sion in an instant.

Ghost isn't really needed, considering how many phantom dancers we have late game.

Heal is a bad idea to get a hold of, first it's terrible late game especially with your ultimate lifesteal.

Any other spell, just wow...

At the beginning of a game, grab a vamp scepter and run up to the bush top or bottom, usually im there at about 0:45, sometimes, some sorry Annie or squishy champion runs into this bush and you have two free hits, then your stun and another two free hits. If they don't do this, they will carefully walk up the opposite side of the lane to get those ever so wanted minion kills, you may not be fast, but you have flash and ignite plus 2 seconds of stunningly great damage. If you have a lane partner like Singed or Ryze that has some more CC, you have an easy kill. If you aren't so fortunate that someone didn't let you have a nice donation of gold early. Find those minions with less than 100 health and hit them, you can get lots of good in not lots of time. Also, you are getting health with every minion kill. This seems so small that it could never make a difference, reality is, with about 150 minion kills, you have around 400-500 bonus health. If you get any good shots at a kill during laning pop your ultimate, you can basically lifesteal a turret by level 9 with this ability.
Teamfighting is quite simple for Sion. You run in, pop your R, leave your E on and stun their Carry/Nuker. Run in, hit the nearest champion 'till it dies, then on to the next one, doublekills and triplekills aren't a rare occurance for me in teamfights. After a teamfight, find the nearest turret and run at it hopefully you will have others to help you but when you get low soloing a turret either kill minions or run out to the jungle and get a few camps for health, 50% lifesteal is an amazing thing, take advantage over it.
I played two games just to give an idea of how easy AD Sion is, these games were my first Sion games in a month or so
A 2.0 KDR might not be great, but remember it's been a while and Sion struggles through early game to harrasses, and I'm not that great of a player in the first place, if I can do this, you can.