I always send Tali, she never dies as long as I keep up with the 8 ventilation controls.
Edit: Tali is a great choice, I sent Legion, as he seemed like the best idea first time playing through. I recalled Shepard telling Ashley that the geth are perfect predators, they don't breathe or feel. I figured he was perfect to send through the vents, especially not knowing I'd need to help him along the way. He survived every time.