Very good heal and nuke use it as needed in lane. In team fights use it to keep people alive.
A nice aura that will keep the enemy form running.
Powerful ultimate, try to save it for team fights to keep everyone alive.
Starting Items
All teams need a courier, if someone else buys it get wards instead.
Good heal if you get low.
Heal to help against harass.
More Mana for when you get low.
Gives some stats and can be upgraded later.
Early Game
Gives a very nice amount of Mana.
Allows you to have lots of Mana as well as give some to the team.
Upgrade the courier when you have some extra money, if its already upgraded, buy some wards.
Core Items
A very good item that will keep your team alive. Your team only needs one so if some else gets one buy wards instead.
Protects your team against magic. Your team only needs one so if some else gets one buy wards instead.
Very powerful item if you can afford it.
Late Game
A good upgrade that allows you to defend.
Very good aura for the team.
A double ult will make your team very hard to kill.
Alternative
Good if you are against enemies that cast a lot.
Good heal for the team and good stats for you. Your team only needs one so if some else gets one buy wards instead.
A good aura and upgrades form a Bracer if you decide to buy one. Your team only needs one so if some else gets one buy wards instead.
If you do not need the Mana form Arcane boots get these instead.
Good cheaper alternative to Scythe of Vyse since it can disarm carries.
A nice aura for the team. Your team only needs one so if some else gets one buy wards instead.
Other Items