Which of these survivors should I purchase first?

Siachi

Using Iridescent Shards. I've got a fair number of them, but I'd still like to budget them out.

SoGo

Renato Lyra

Probably Renato.

Yun-Jin and Thalita have useless perks.

Taurie has one great perk - Shoulder the Burden, and two not so good ones.

Renato has two really nice perks, and his teamwork perk, which is eh.

Linkdouken

Yun-Jin Lee

Yun-Jin. She's a nicely designed character with decent cosmetics and her perks are quite good. Smash Hit can grant you decent distance at an otherwise weak pallet if you can get the stun

IzofinChoyzei

Depends on your playstyle

TheSingularity

Renato Lyra

Renato.

P0rshulz

Taurie Cain

Personally, I like Taurie's perks, I find them better than Renato's and Thalita's and especially Yun-Jin's, though, it depends on the way you play.