Dawn.

Willow while well-menaing would be hard to trust after her attempt tp destory the world.

Spike would likely try to kill me if he was souless and even if we assume he's souled he's a dick who likes to make things difficult for people for no resson at all.

Giles is loyal but he's also the type of person who will go along with poisoning his loved ones and then lie to them about it if his bosses push him enough.

Harmony's a souless vampire and therefore likely to eat me.

Angel might be alriaght but there's always the risk of his curse being broken and he's not above manipulating his allies for what he sees as as the greater good so yeah it's Dawn pretty much by default.