Answer: Dallas
In the theatrical release, Dallas' death was not shown; rather, his last moments were inferred by a quick shot of the alien reaching for him and screeching at him, then Dallas' radio going dead.
The 25th Anniversary Director's Cut of "Alien" includes an extended scene where Ripley located Dallas during her escape to the shuttle. He has been "cocooned" by the alien, and, while still barely conscious, begs Ripley to kill him. She obliges with her flamethrower.