I'm new, and this is my first build I've created on mobafire, so we'll see how it goes. Now, I started playing LoL a couple months ago. The first game I played was with Sion, on Twisted Treeline. I somehow won without knowing how to do anything and trying to use smite on players. This is my second attempt at it.

For some reason I feel like this should be a no-brainer build-esque build, but most Sions seem to just be those atkspd-dmg-tanks. Here it is.

Let's hope it brings about something productive.

Okay guys, I am a total noob at these, but these were a weak attempt at it. I do think that the movement speed quintessence is important for being able to run around early game and to be able to place your Shield Bombs correctly though. =] Cooldown reduction is mostly important late game, once you have the cooldown on your stun lowered, although it is important all-around according to me.

Alright, I go 10 0 20. Now I don't really care to take the 9 0 21 route because I don't really need it. Ghost works just fine usually when I need it to without it, and ignite is a low cooldown already. I really do like the cooldown masteries however, and Sion can get mana-hungry. The monster-buff masteries are taken because Sion's mana isn't that great, and the blue buff can really fix that problem. The ignite buff and spell penetration are great as well, and if you wanted, you could opt for 9 0 21 if you wanted a different spell, like say, Teleport or Flash [which you would want the -15% CD reduction], but personal preference still greedily draws me to the true damage and +10 AP.

Start with a Doran's Ring. I choose Doran's bring because of the extra survivability, the the mana regen is a lifesaver, and extra AP is just icing on the cake.
Lane.
By level 4, you should be able to start killing the enemy easily with a shield-burst-stun combo with your partner [they are invaluable when you can stun the enemy for two seconds a pop].
Buy Sorcerer's Boots as soon as you can, need that speed, and Spell Pen. is just obvious.
Next, go for a Sheen and stun-bomb-crush people. Upgrade it to a Lich Bane when you can.
By now you'll be able to pack a bit of a punch, along with a couple of kills, but surely not as much punch as you'd like. Rylai's provides a good amount of OOMPH and AoE slows, along with a nice cushy Rabadon's to pack that extra 200 damage. Teamfighting should go smoothly, picking off squishies and carries with your stun and your team. Coordination is key.
Now, we're looking for some cooldowns. It's probably late game, so Nashor's Tooth provides cooldowns, ability power, and attack speed, which with conjuntion with your ultimate, should prove powerful. You should probably be able to spam your abilities right now, so it might leave you a bit mana hungry. The blue buff should solve most of your problems though.
Now if your game has dragged on this long, then sell that doran's ring which seems to be growing more and more useless. Zhonya's Hourglass gives you an extra 100 AP and boosts your armor up higher. GG.

UPDATED 3/5/11: Disregard nashor's tooth and rylai's. Now, depending on the situation, after you build your Lich Bane, you'll want to measure whether or not you run the risk of being killed by DPSers or AP. So you build accordingly.
Zhonya's for DPS, Abyssal Scepter if not. After you build either or both, depending on your situation again, build Rabadon's, and your AP scales high. Top your b uild off with Morello's for an extra 70 AP and20% CDR.

In the early game, you'll want to farm, which should be easy. Don't be stingy with using your shield, and when you have ignite, use it. A strategy that I use it to hide in the bushes, press your W [Shield], and wait until you can trigger it yourself. Your damage is enough to make a dent in a lot of builds. Sion is also a fairly adept meleer, although not for very prolonged amounts of time. His damage is great, but his attackspeed leaves him at a bit of a loss, so it's easiest to simply play him as a caster.

In mid-game, there are exceptions, however, particularly when you get your lichbane and sheen. Stun-ShieldBurst-Hit combo is very devastating, and will often drop people a good amount in one burst. Another tactic is to use your shield to tower dive. Your AP blocks most of the damage from a tower, and a stun-combo can knock someone out without putting yourself in much danger.

By late game you should have amazing cooldown reduction, and you will almost be able to spam your Q-stun, having around 5 seconds cooldown, 2 of those seconds they're being owned by you and your friends.

While you CAN be very ballsy with Sion, combining his passive and your W along with your Enrage ability to get great HP. It is a good idea to stay away from nukes however, as your MR and Armor are not the best, although if you've been farming right, then your HP should be awesome. Your W should be able to take out an entire group of spawns with relatively low cooldown, and by late game, while you're gaining 3 hitpoints a kill, farming when the count is down is not too bad an idea.

Overall, I built this Sion as something pretty straightforwards to me,it's simple enough. An AP Tank, something that seems to not be around as much as the pool of AD/DPS sions on the forums.Well, I hope this build suits your interests, it works for me. I'd really like any feedback though, ESPECIALLY concerning runes and masteries. Peace.

Crimmage, out.