Wow this is a fun topic. I couldn't just narrow it down to just one actor for each position.
Captain: (Bruce Willis or Liam Neeson)
First Officer: (Dennis Haysbert or Aaron Eckhart))
Engineer Chief: (Robert Downey Jr, Amanda Tapping, or Sergio Di Zio)
Chief of Medical Officer: (Dana Delaney, Elizabeth Mitchell, or William Fichtner)
Security Chief: (Chris O'Donnell, JD Pardo, or Jeremy Renner)