- Aghanim is your core. It allows you to use your ult in every teamfight and turns it from "Ouch, this hurts" into "THE PAAAAAAAAAAAAAIN THE PAAAAAAAAAAAAIN". Remember that people cannot die from it, though.
-Wards too. Pretty please.
After that, every single item you can buy is up to you. Some notable options are listed below, but if you think you need a heart of tarrasque, there's no problem in going and buying it.
- I like not upgrading my boots until a bit later in the game, and then getting travels, but going tranquils, arcane or even treads is legit.
-You'll probably won't be allowed to just waltz into the enemy team and nova them, so something like blink dagger or shadow blade can help you drop it. I'd go with blink, but both are legit.
-If you are getting rightclicked hard, buy a ghost scepter
-If you need raw hp, get a mek. Just remember to activate it.
-If you want more damage early game, get a medallion. Your carry will love you.
-Hex, orchid and halberd if you want disables
-If you don't know what to build, get drums or a force staff
-If you want loads of single target damage late game and pushign power, get necronomicon
-If you want for your ult to be even more damaging, get a veil.
-If you have extra money and want more survivability, get shiva's
-Get vlads late game if you have a melee carry and he hasn't gotten it yet. He'll love you.