from a potential standpoint, what thief wiuld you say is the most dangerous/powerful?

Depends heavily on the context- A Thief of Hope or Life, or a Thief of Mind/Heart may be more dangerous to people, but a Thief of Space/Time or a Thief of Light/Void would be more dangerous to their environment, to SBURB as a whole, and to Paradox Space.

I’m going to say a Thief of Light simply because we’ve seen the fuckery Vriska can get up to.