I play Sion as a jungler without any standard jungling items as I never find a need for it. This Sion build is focused on critical damage and attack speed.

Pros:
-Lots of health during late game.
-Over Powered early game.
-Can last in Ganks while using Ulti.
-Stuns follow the person.

Cons:
-Not much attack speed early game.
-Hard to Jungle without smite.
-Cannot handle a 1v3/4/5 unless you were fed

Why mostly AD runes? It's important as you really won't need to buy any attack damage item before completing your Phantom Dancer (Except Doran's Blade). AD runes will help in early game especially in jungling (your attack damage will be 115 when you're level 1).

The Masteries is made so that you get full AD and can walk fast (because Sion will chop heroes down slowly). The Masteries will give you enough damage till you can get a phantom dancer.

It is very important to farm with Sion as you'll get permanent health each time you kill a minion. This means you can get lots of health if the game lasts around 40-50 min. However do remember to help gank lanes instead of just farming.

For obvious reasons, take smite and flash. Also Smite works with your 3rd skill's passive so it enables easy farming. Jungling without Smite is very hard. Because you cannot burst damage or have any extra skill (your first skill doesn't really count as a skill as it helps only with basic attacks) during early game.

You get lots of attack speed and critical and you wont need much damage. Also with a PD and IE you can easily push lanes. Attack speed and Critical will help a lot when you use your ulti (which gives 100% lifesteal 50% attack speed and heal nearby allies for 50% of the damage dealt). Which means you can survive through ganks and "support" teammates. I tried building full AD Sion but He attacks too slow to heal teammates quickly thus the need for Attack speed items. Get a Warmog as your last item if you need to survive more longer or be able to tank. Get a Bloodthirster if you don't need to tank for anyone as it gives you 100 damage and 25% lifesteal which helps with your ulti.

Start of at Blue Golem, Wolfs, Wraiths, Golem then go back to heal and get your shoes. Always rememeber to use Death's Carress and make you sure you detonate it for some damage. Always smite when possible. When you return, finish of the Red Lizard. By that time, Wolfs and Wraiths should be respawned and finish them off. Once you are Level 5, start ganking lanes. When you are Level 6, it is possible to solo 2 heroes attacking you (unless you get exhausted or any CC).

Use Death's Carress and Cryptic Gaze to kill of a single target. You should always engage with your shield on and stun the carry. When you start to see your teammates health low (which is about 2-4 seconds in a gank), use your ulti. This will enable them to have more survivability. Try not to engage without your ulti as it would be risky.

Ganking without stun is possible but only if you are extremely sure that you can kill the opponent hero. Or at least have someone that can CC the opponent.

Penta kills is definitely possible but sometimes at the risk of your teammates.

Sion is one Over Powered hero and can be extremely deadly in Ranked Games. If you can master him, he would be able to give you and your teammates the upperhand in ganks. Also, try to farm whenever you can, near carries because you'll need to be present in ganks. Try to aim for 200-350 CS for a 40 min game as it'll give you lots of health.