I picked Alexandra, since she started it i felt she should have ended it.
followed by Joan and Jonathan
Those would have been my sentimental choices, too.
But storylinewise, having Keith Prentice/Morgan, who was also in the episode, do the final voiceover might have made more sense. Still being very much alive, Ben (or Bramwell or Flora) could have been looking back from some perspective in the future - but that wasn't actually made as clear as it might or should have been. However, as has been an often used literary device, Morgan [spoiler]in death could have been aware of future events,[/spoiler]and he would have been capable of laying them out exactly as the final voiceover does. Though, of course, I realize that the very idea of Prentice/Morgan doing that final voiceover would be positively horrifying to some.
Interestingly, I seem to recall reading that the closing voiceover doesn't even appear in the final script. It was something that was decided upon at some point later on after the script had already been completed.