My don't-trust ranking:
1. Gale, not a sincere type, it's not him been evil, it's him not trusting others, who knows what else condition he has?
2. Shadowheart, one sorry soul who doesn't even know her own past;
3. Astarion, he is only trustworthy if you and him sharing the same goal;
4. Wyll, hypocritical yet daring, even was blind by vengeance, still kind of predictable;
5. Lae'zel, probably an exile of her kind, has strong discipline which makes her reliable.