1)The jungler Item:
Some people build the Feral Flare instead of the Spirit of the Ancient Golem because Sion has to farm a lot in early game. I think that the offensive items that I've chosen for Sion can't be changed and with just 2 defensive items he could be an easy kill (it's not bad to die in the teamfight, but at least live untill they used their ccs, or your passive will be pretty useless). By the way, the Spirit of the Ancient Golem is just op on Sion, thanks to his 25% on health multiplier, which stacks with your passive. At the end of the build, if you farmed well, you could also have more then 1000 hp bonus... not bad.

2)The Shoes:
This is a hard choice: Boots of Swiftness or Mercury's Treads? Analyze the enemy team's composition and do your own considerations. Are there ccs which gives you pain? If you can't do anything about that, buy the Mercury's Treads. If you haven't got problems, buy Boots of Swiftness, obviously better then the Mobility Boots, because you don't need the movement speed to engage, but to chase down the enemy carries.

3)The Tiamat and the Giant's Belt:
The Tiamat is usefull to clean the jungle faster without spending too much mana. You don't need to close the Ravenous Hydra yet, you just need that AoE, at the moment. Remember that you can use the active when you are is your passive form, it will help you a lot. The Giant's Belt gives you almost 500 hp with the Spirit of the Ancient Golem, a good deal if the lane phase is over.

4)The Youmuu's Ghostblade:
This could seem a gamble, but I think that it could be a core item, in this Sion build. How could the Sion's passive ever work without the Youmuu's Ghostblade? Its active is essential to chase down the enemies: you are faster and your attack speed is increased! This means that you can give more basic attacks, which mean more life stolen, which means more time in the teamfight, when you are dead. This item can change the sort of a teamfight, so use it properly! Use the active only if you are dead or if you know you will not die during the fight.

5)The Tankyness:
Every game is different and the situations can be a lot. The Randuin's Omen can be a good deal. Always remember to use its active to chase the enemy! The Spirit Visage gives you more regen and lifesteal and it works very well with Sion, but it is not always the best item: the Banshee's Veil's passive could help you in other circumstances. The Warmog's Armor gives you 1250 hp... not bad! It is not always worth anyway, if you need resistances, but is usefull against true damages.

6)The Ravenous Hydra:
Finally, you can take it! You can also do it before, if you are ahead.

7)Other possibilities:
The Atma's Impaler is good, too, if you started with a full tanky build and you need a good and not expensive damage in late game. The Scrying Orb could be nice, to see the enemy carry before engaging.

Sion is probably not the best ganker in the game pre-6, but with a bit of strategy, he is more then viable.

Obviously, start charging the Q at 1:53 at the first buff, to gain 2 seconds on the enemy jungler. Don't be too scared at the second buff: if the enemy jungler comes to kill you, he will die with you, probably (pay attention to junglers like shaco, lee, etc., who have got nice escapes).
Gank top at level 3: ward the river, slow the enemy top laner and charge the Q for 1.25 seconds, don't try to max. your damage. The enemy will probably flash away, but it will be worth: your ward will save your top laner from a certain enemy gank.

Farm in the jungle and try to reach faster the 6. If something happens, Sion has a good counter gank power.

A Q->W->Q start is also good, if you don't wanna gank.

The ultimate is what told me that Sion is a jungler, not a top laner. Because he is stronger as a jungler? Maybe he is not, but he is just too funny with his level 6 ganks.
Ok, the ultimate is hard to land everytime, but there is something you can avoid to prevent the missing.

1)Sion doesn't gank from the river: the 95% of the times it is worth to gank from the lane, or you could simply go fast into a wall or to the other side of the jungle;
2)Ping several times before ganking: even a little slow could help you to land your ultimate;
3)Be faster, be happier: look at your range and try to maximize your speed and damage;
4)Please, don't hit your own tower, pay attention;
5)If everything fails, press R: When you understand you can't hit the enemy, press immediately R again, to stop as soon as possible your ultimate (the animation is very slow, so it could be the end of the gank, but at least you will not run untill the enemy tower).

Well then, if you land the ultimate, the kill is done: Charge your Q, activate your shield, slow the enemy and let the shield explode to deal more damage.

Here you can take advantage of every aspect of Sion!
You are the engager, with your ultimate, but look at your team before engaging, don't go too deep! If it's easy to fail the ultimate during a gank, it is not, to open a fight. But what do you need is to reach the carries, so try to dodge the tanks and go straight to the AD and AP carries (again, if they are too far, don't engage in this way if the team can't follow, or you will just 1v5 them). If you die during the process, go ahead! Use the 3 usefull active that you have and kill them all!

This is my first build and it is not completed yet, so give me a feedback and comment, this will help me a lot! Thank you and Good Luck with Sion, Summoners!