Lewis Zimmerman, if just because Robert Picardo's portrayals of both him and the EMH (and how much they bounced off each other despite having so much in common) was always a delight to watch.

Picardo once talked about a pitch for a show he had floated to producers a couple years back (he apparently didn't get any bites) that he jokingly called Star Trek: Picardo, in which was a show that reunited the Voyager EMH with Zimmerman in a season-long arc that had them having to work together again, and it just sounded like so much fun I'm kinda sorry it didn't go anywhere.

Mirror Spock, hands down.

@Hurga

Yes!

Zimmerman