I've racked up a number of Sion top and one mid game of late.

I've been running him 21-0-9 and he still feels plenty tanky enough most of the time.

I've been using two rune pages for him depending on my lane opponent.

Vs AD - 9 Hyb Pen, 9 Armor, 9 Magic Pen, 2 MS Quints + 1 Hyb Pen Quint
Vs AP - 9 Hyb Pen, 9 Scaling Health, 9 MR, 2MS Quints + 1 Hyb Pen Quint (grrr...Teemo).

Sion's main form of harrass in lane is Roar of the Slayer. It does silly damage if you manage to smack a minion into the enemy champion, especially given all the magic penetration you are sporting with the runes mentioned above.

Teemo - You have to pack some MR into your build when your are vs Satan otherwise he will kill you early game. Mid - Late Game is a different story. Once you get Unstoppable Onslaught kill that annoying yordle every time you see him. He is squishy and can't escape you without using flash and you still can get him if you play cagey and get him to pop his flash before you ult.

Jayce - is squishy like Teemo. Just buy a Frozen Heart, dodge his Shock Blast (the 6% MS from Quints and Masteries really help with this), and harass him out of lane or kill him if he overstays.

Maokai - Farm fest. You can't kill him without help and he can't kill you without help. Just don't overextend before you get your ultimate as he is great at setting up ganks with Twisted Advance. He can also interrupt your Decimating Smash channel with Twisted Advance or Arcane Smash. You can setup ganks on him when you hit 6th level, but it is still hard to kill him unless he is really out of position and your jungler brings a lot of damage because of the damage reduction from Vengeful Maelstrom. Your best bet for picking up kills and assists are by doing Teleport ganks when your ultimate is up, so keep an eye on the mini-map for those opportunities.

Gnar - Harass the **** out of him with Roar of the Slayer when he is in mini-Gnar form and just stay away from him when he is in Mega-Gnar form. If he overstays while on low health, ult him and kill him.

Problem children:

Mordekaiser

Malphite

Both have shields that refresh, which can mitigate a lot of your harass.

Of the two, Malphite is probably the harder to deal with as he is also going to build tanky so it is hard to get him very low and he can just peace out with his ultimate if he does get in an awkward spot.

You can get Morde low if you harass him when his shield is down and then have your jungler come in for a gank. But once he gets Hextech Revolver this gets harder. Oh if Morde uses his ultimate Children of the Grave on you he only gets a clone of you after your passive wears off so if you or someone else on your team can kill him before that happens then you're team won't have to face a ghost of yourself.

Game Start:

At level one you can take advantage of your passive to kill the raptors (you take these because your jungler won't want to go anywhere near them until at least their second clear because they flippin' hurt) to get level two before your opponent and then teleport to lane.

I've only successfully pulled this off by leveling Decimating Smash to start. Tried it with Soul Furnace and the raptors tore through the shield too fast. Awkward.

Get the raptors lined up so your decimating smash will hit them all and then power it up at 1:53. That way it will be fully channeled when they spawn at 1:55. Kill all the little raptors and then focus on the big raptor. Decimating Smash should be off cooldown so you should be able to get one more off before you die. After dying finish off the big raptor. At this point you will be level two so level up Roar of the Slayer. You may have to wait a few seconds for your temporary health to drain out and then you will be back at the base. I usually pick up a Pink Ward and then teleport to lane. Once you hit level three remember to level up Soul Furnace so you can start stacking health for every minion and champion kill.

If you have good vision on the enemy team you can also take the buff that the enemy jungler is not starting at, but this is definitely riskier than taking raptors because if you are caught before completing the job your killer will get credit for first blood.

I don't know this for sure, but I think the enemy champion would have to deal the killing blow to you before your passive comes on line otherwise the buff would get credit for killing you not the enemy champion.

Items:

So far I've just been building pure tank for the most part. Since Sion passively stacks health after leveling Soul Furnace he doesn't really have to go directly for items that provide health unlike a lot of other potential tanks. Such being the case your first purchases should be geared toward dealing with your lane opponent. So Frozen Heart for AD opponents and Spirit Visage for AP opponents. Glacial Shroud and Spectre's Cowl are both great early buys that won't break the bank if you are CSing reasonably well. Warden's Mail can be picked up first if you are against an autoattackcentric laner like Jax or Tryndamere though the 10% CDR from Glacial Shroud is really nice for faster uptime on your Roar of the Slayer.

Zz'Rot Portal - Sion can actually push minion waves fairly fast and he can get out of Dodge with his ultimate if things start to look ugly so splitpushing is an option with him. However, you are a tank with a lot of CC so it is usually a good idea to be with your team in team fights and since you are building tank you don't have much AD so killing towers by yourself takes forever, which is where Zz'Rot Portal comes into play. So once you grab this item, pick up some wards, a sweeper, and make sure your Teleport is up and the go pick a side lane to push. Top if dragon is up or about to spawn. Bottom if you think you might be doing Baron soon. The reason for the choices is that the Zz'Rot portal is vulnerable if located and people are likely to be swarming around the dragon side of the map if it is up or the Baron side of the map if the enemy team thinks you are planning on doing Baron so the portal is liable to be found and killed quickly. Also by pushing the side of the map where no one is defending this creates cross-map pressure (e.g. someone on the enemy team has to go a long way out of their way to deal with the threat).

Okay with all that out of the way you can either wholesale push the enemy waves up to where you want to install the Zz'Rot Portal or if you are really industrious you can set up your minion waves to slow push to the enemy's base by only killing off the caster minions in the next two to three minions waves and then install the Zz'Rot portal. What this does is causes your minion waves to stack up due to pausing to take out the leftover melee minions in the enemy waves (your minion waves will start catching up to each other). Between that an the Zz'Rot portal you will create a real threat that someone will have to deal with if they don't want to lose a turret.

If you look around online, you'll find some posts with maps that show good locations for placing the Zz'Rot portal. (I may try to find one and post it later). Because the Voidspawn it produces will only travel so far before expiring automatically whether they kill or damage anything or not. Make sure to use your Sweeping Lens / Oracle's Lens to check for wards in the brush where you plan to place the Zz'Rot portal before plopping it down.

Sorcerer's Shoes - If your lane opponent is building MR you can build these to try and further cut into their MR above and beyond your Runes and Masteries. Overall, I usually find CC to only be a minor annoyance to Sion which is why these work ok.

I say ok, because if you aren't already killing your lane opponent without them, they probably aren't going to make that big of a difference, so you may be better off buying boots geared toward dealing with the damage type of any enemy laners that are getting fed. Optionally if the game goes long enough you can swap them out for boots that make you tankier, though I would only do this very late game as it is usually cost inefficient to sell a completed item for another.

Thornmail - Generally my go to item for dealing with fed AD carries after Frozen Heart. You could rush this after Frozen Heart to really make the enemy Tryndamere crazy.

Warmog's Armor - Actually a really nice end item (once you already have items that provide resistance to damage to protect your health) for Sion as it increases the strength of his shield from Soul Furnace making him harder to kill and if you do die between the health regeneration provided by Warmog's and Spirit Visage and the lifesteal from your passive, Glory in Death, you can remain reanimated for a very long time and continue to dish out percent health damage with your autoattacks.

Pie in the Sky:

It is a sad Sion who dies and gets CCd or kited preventing him from getting revenge on his killer(s) via his passive. Seriously, his passive feels rather underwhelming at times because of that possibility.

If I were to make one change to his kit I would increase the bonus movement speed provide by the active portion of his passive, Death Surge, to something like 50% + 10% of his bonus health and at least make him immune to slows during that time. Pesky CC need not apply. Noxus!!!!!!!