There is so op build that if you can build this build in the game you never going to lose. This build will pay much, so you have to last hit and farm minions so much as you can. If you are playing 5vs5 you need to go top solo or mid.(top is better)In 3vs3 you need to go top and last minions as much as possible.

1x Boots
2x Phantom Dancers -sell boots if you want buy two panthom dancers
1x Warmog's Armor
1x Infinity Edge
1x Bloodthirster
1x Guardian angel -(you can also buy atmas if you have much hp and you will survive tf)
those items are my most common choice. But if I am trouble in beging I will buy those def items earlier and if they have good magick dmg team I will buy more mr for example force of nature.

Buy crit runes or armor penetration runes. It is your call. ( crit runes are best choice if you want dominate early game)

In team fight you are your team tank so you need to go fight first and try to stun their team ad or ap carry and kill it. You can also 1vs1 almost everyone champion and win that fight. Just put your ultimate on and start hitting, but you need to care exhaust or ignite becouse they will stop you livesteal.

Just last hit minions never autoattack because then those minions are pushed and you are easy gank. Of course if it is save and you enemy is going back to base push like hell and try to take tower down or just try push minions theyr turret so your enemy can't get xp.
3vs3 I last hit minions on top and I am trying to harras my enemy. If minions are pushed too far just go ganking others or go take jungle minions.

I like those ghost and exhaust, but good spell to sion is flash. It is you choice if you want to play exhaust ghost or exhaust flash.