Most standard AD Sion builds focus on heaping on a bunch of straight AD but I've come to realize that you don't really need it. Sion gets plenty of AD from Enrage so as long as you have enough lifesteal to counter the health cost you can pretty much focus on other stats. Plus, who doesn't like seeing all those little red numbers flashing above their enemies heads every time they fight?

EDIT: Slight quality edits and a bit of extra info.

Pros:
-Fun as hell
-Insane DPS
-Relatively high survivability even when built mostly Glass Canon
Cons:
-Inconsistent damage till you get your crit chance up
-Vulnerable to CC

I tend to run Crit Sion 24/0/6 so I get maximum damage, a bit of life steal and a smidge of utility on the other side. Really though, you can run just about anything you like (though I highly recommend getting the Lifesteal mastery at least, it's quite useful at lower levels for ameliorating the hp cost of Enrage).

I tend to run Flat Armor Pen Reds, Flat Armor Yellows, Flat Magic Resist Blues and Lifesteal Quints (Not pictured above because they don't seem to be in the system yet... :( ). Crit Damage, Crit Chance or Armor Pen Quints work too as, likewise, do Crit Damage or Crit Chance reds. Scaling runes are almost never a good idea because the small bonuses runes give are of best use at the start of the game. By the time scaling runs are actually better their effect is basically nil anyway.

Your priority should be REWQ or REQW. You want to master Enrage first because you're not getting much AD from items till late in the build so you'll need it to actually do damage. Get points in Cannibalism whenever you can, as you would with any champion, especially since Sion has one of the best ults in the game. Whether you max Death's Caress or Cryptic Gaze first comes down to personal preference, either is a perfectly valid option. I personally tend to go with the shield first for a bit more survivability but it doesn't really matter much in the end.

YOU! MUST! GET! Ghost! Not an option. It's one of the best spells on dominion. The only time it's not mandatory is if you're taking bottom lane and even then it can be helpful. For you second spell I like to take Garrison or Promote to help cap points but really it all comes down to personal preference. Take something that suits your playstyle.

Start with Berserker's Greaves and Vampiric Scepter, for move speed, attack speed and a bit more life steal to offset Enrage's HP cost. Head for The Windmill with your team, popping ghost when you get out of the lane and passing through the speed shrine on your way up. You'll get there faster if you hit the speed shrine when you have a bit of ghost time left than you will if ghost runs out before you hit the shrine. If they have Rammus or some other speed demon who managed to get there before you or they just get there at the same time as you try and contest the point till your team shows up. Try not to die too quickly though, you won't help cap if you're dead. Don't be afraid to sacrifice yourself if it it'll secure your team an advantage though. It's hard to say when retreat or sacrifice is the right choice, it comes down to experience. Don't fret if you get it wrong though, it's easy enough to recover from minor setbacks on the scar...

First time back to base you want to turn your Scepter into an Executioner's Calling, or at least get the Brawler's Gloves for it. This is a great item for this build because it provides crit chance, a good bit of lifesteal, a slightly weak passive that we don't really care about and a wonderfully spammable active that's quite useful for putting down anyone that was counting on healing themselves. Make sure you use it at every opportunity, it's on such a short cool down there's no reason not to.

At this point your goal is to hold at least 3 points for the rest of the game. Windmill is the most important. Ninja cap undefended points and push waves of minions to cap defended points. Do go after quests. Don't go bot unless your bottom laner asks for help or there's a quest down there. Nothing annoys a bottom laner more than idiot teammates who don't realize that it's a solo lane and cause your team to lose windmill over and over again by insisting on being where they're neither needed nor wanted.

Your next item after Executioner's should be Zeal (or either of it's components) which you'll want to build into a Phantom Dancer ASAP. This provides a nice chunk of crit chance, attack speed and move speed (all of which you dearly want in this build).

After Phantom Dancer your options really open up and it all comes down to what you want to do and what the enemy team's doing. I've put two different builds up there, the first is the one I usually use, the other gives some alternate options. I'll talk a bit about some of your choices and why you'd make them now:

Cloak and Dagger: Crit chance, Attack Speed and Tenacity. Great choice if the other team is CCing you a lot and decent if they're doing it at all. If the other team is light on CC or isn't using it effectively there are better choices though.

Infinity Edge: It's quite expensive and there usually isn't that much game left by the time you build it but DAMN. The extra crit damage stacks with runes and the extra crit chance it gives will generally put you somewhere in the neighborhood of 90% crit chance so you'll be doing it on every hit. Add in the AD it gives and this is a beastly item, no doubt.

Youmuu's Ghostblade: CDR to spam your skills more, Crit Chance, armor pen, a *****in active and a recipe full of things you can use. What's not to love?

Black Cleaver or Last Whisper: Get one of these if the enemy team is stacking armor. Black cleaver is more effective till they get a ridiculous amount of armor, especially if you've got teammates that do physical damage you can all focus on one target together. It takes a couple of hits to get going though, and Last Whisper penetrates more if you're dealing with a Rammus or someone else who's just heaping the armor on.

Atma's Impaler: Armor, Crit Chance and the ability to turn your HP into AD (which you should have a fair bit of even if you built none courtesy of Enrage). An excellent item on this, or really any, Sion build.

Most of this build's survivability comes from the insane lifesteal you get when Cannibalism is running (and the modest amount of it you get when it's down) but the thing that separates good players from great is knowing when to counterbuild. If the other team is stomping your face in you should take note of who's doing it and what kind of damage they do and buy appropriate counters. Some good choices are:

AD/Attack Speed: Frozen Heart. The CDR makes cannibalism even more spammable than it already is and the Armor and AS reduction shut down auto-attackers cold. One of the best ways to make a Vayne that's ****ing with you cry.

Magic Damage: Force of Nature or Wit's End. Wit's End is often overlooked but it provides bonus on-on hit magic damage, attack speed and an excellent amount of MR for the price (especially once you get the stacks up). The MR, MS and regen on FON basically makes you unkillable by a magic heavy team.

Lots of Stealth: The Lightbringer. None of it's stats are useless and it'll really ruin any stealthy champion's day with that passive. Oracle's Extract will, obviously, work as well.

Other Good Choices for Counter-building:

Sunfire Aegis: armor or magic resist works much better with health backing it so getting health and armor in one package is just good efficiency on your part. The passive AOE damage is quite nice as well because you're going to have to get in people's faces anyway.

Thornmail: Godly against a team full of auto-attackers, not that great unless at least 3 or 4 of the other team are right-click and win though.

Traditional AD: A solid option and quite fun. A lot of the itemization I've provided works just as well there as it does here.

AP: No. Just no. AP Sion is trash and every time I see someone build him that way it makes me cry. All he is is a stun and a shield at that point. Don't get me wrong, he does decent burst and that shield can soak quite a lot of punishment up (and you're a lot more likely to be able to pop it for more damage because of that) but he's basically a one trick pony and it completely wastes the potential of his ult.

Hybrid: Hybrid Sion can be quite fun to play. I recommend Trinity Force, Guinsoo's Rageblade, Hextech Gunblade and whatever else you want (though try and focus on AD to take advantage of that sweet sweet 150% bonus damage from Trin. Force though).