This short guild is me trying to make a sense of all Sions abilities.
Synergy isn't really there but I think this little "guide" shows some items that can make use of all his skills and competitive at the same time... I think.

The one big downside to Sion (I've felt) is his range and his slow movement. So thats another point of focus.

Runes I wont explain too much. He needs mana. He needs mana regen. He needs to be fast and he needs to hurt. Thats pretty much it.

As for masteries I feel your own personal style is best. I tend to really like a faster more mana-heavy sion so I dump 19 in Utility (emphasizing on and for Ranked games) and enough in Offense to gain max Sunder. But like i said, its personal preference.

My skill progression is simply based on snowballing. I want to maximize his heath from to boost DPS from mid/late game.

Summoner Spells based off the mastery choices are obviously going to be and .

and are my choices. Keeps you in lane longer and with the Rune / Mastery set-up your mana will not be in that bad of condition. This also alows you to start building your damage with sooner with less risk.

Philosopher's Stone Should be built as soon as possible

Should be built next. Movement and Attack Speed by the time your are level 6 is a good thing.

Catalyst the Protector for early/mid game survivability and helps keep you stay on the field longer while farming/leveling.

or next. Followed by the second choice. CD reduction is important.

By now you should really be reaching late game. Here you want to complete items you started and make final

for stun protection and defense.

or . If you want to finish off your CD cap then Nashor's but if you think it can wait i suggest getting your and then Phantom Dancers.

To finish things off. By now your max HP should be 3.5k M/L and nothing should stand a chance.