Rein listed six starting item builds for supports in Dota 2

The most OP build right now is for right-click supports (Bane, Wyvern, Phoenix, V.D., Silencer, Jaggernaut, Mirana). Three Tangos is always risky; with the Bounty, buy one Salva and a Mango—that should be enough for the entire laning phase, and maybe even another pack of Tangos. Don’t take Clarity during the laning phase; Mango > Clarity.

A 6-Tango build—you can use this in any game against any ranged hero. Use the Bounty to buy Mango; if the lane is tough (Ax, Necro, Underlord, Doom, Summoners), ideally buy a Salva as well.

Build for casters: Lich / CM / Disenchanter / sometimes works
on Bane. After Bounty, get Salva, then save up for a Stick / Boots.

Build against caster teams. I usually go for this if the enemies have low-cooldown spells on two heroes, for example: Mars + Belka, Tide + Rubik, Bristle + Kotl—you’ll figure out who to target based on experience. Don’t take this against spell-less support heroes (Pudge, Bara, etc.); you shouldn’t take it against Lich, Pudge, or Doom 3 either—stats are better. Back at base, buy Bounty Salvo + Mango

A build with a Fairy and 1 Sentry + Stick, if your lane is stronger and you don’t need to ban Big. This is an option worth considering only after you’ve fully explored all other builds.

For the most resourceful players who buy a Circlet into Urn / Bracer / Null, try it at your own risk (the screenshot shows 1 Sentry and 3 Tangos)