Probably Vex. They can think, and they're only [i]mostly[/i] synthetic, but you can't reason with a machine and it's purpose.
Any other race can be persuaded and manipulated.
English
-
A vex death seems like it would be less horrifying than meeting your maker at the hands of a thrall
-
They're analytical though. Who knows what kinds of things they'd do to you to get some data. Spinal fluid extraction, colon inspection, and god knows what else. They might dissect you just for research.