I'd trust Twilight Sparkle the most, only because I can easily identify with her, and she's in a way the "leader" of the mane6. While she can have her crazy moments, they're usually only a result of her OCD in regards to accomplishing something. We've also seen several past situations where Twilight was doubted by all, but in the end was mostly right.
Rainbow Dash, however, I'd trust the least. Her action-oriented personality makes for an unpredictable character. Not to say she's corrupt, or not trustworthy in the least, but I'd always be watching my back around her.
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