In conversing in other threads, I've mentioned that I think Robert Picardo, throughout Voyager, has demonstrated himself to be potentially one of the most diverse actors potentially in all of trek. He certainly is a contender for the "nomination", but in terms of Voyager, I think the writers certainly gave him the ability to absolutely flourish as an actor and that most certainly is contrasted on screen as we see him evolve over the seasons.
There are several landmark episodes I think really drive home his talent as an actor:
Heroes and Demons
(S01E12) - This is the first episode I feel we really start to get a hint of the potential of Robert Picardo as an actor. Over the course of this single episode, we watch his character evolve and flourish on screen.
(S02E03) - In this episode we watch as The Doctor struggles with what reality is to him. Contrasting to other characters who we've seen go threw this fight on screen such as "Remember Me" (TNG S04E05), "Distant Voices" (DS9 S03E18) or "Doctor's Orders" (ENT S03E16), Picardo commanded the role. Ironically each of these instances show cases the doctor from the series but I think Picardo is objectively the clear winner with Billingsley being a close second. I won't deny a lot of the great potential for this sort of story came from masterful scripting, however.
(S03E14) - This episode clearly illustrated Picardos ability to jump into wide ranges of personalities and illustrated a grand command of persona. Everytime I rewatch this episode, I'm reminded how deeply Picardo can pull off the range of these parts.
Message in a Bottle
(S04E14) - I personally find this to be one the series best episodes and a lot of comes down the execution of Picardo's character and the illustration of his character's adaptability in a tough situations.
(S04E23) - This episode was nominated for an emmay sheerly because of Picardo's acting in it (I don't recall nor can find if it got it, so I am kind of forced to believe it didn't get it). Picardo's character fought so terribly hard for the honor of his ship mates you felt an absolute sympathy for his situation.
(S07E24) - I'll jump ahead to this one to keep the post a little on the shorter side. Though I'd concede that a lot of this episode comes down to writing for the character over writing for the actor, this episode was such an awesome second to last episode. Looking back at all the other series, only Enterprise has a more memorable second to last episode.
Looking back on the totality of the episodes that Picardo really made shine and the episodes that he headlined and turned into grade AAA episodes, often times I'm all but forced to consider him as the most important actor of the series as a whole, after you take everything else into account. That's not to say other characters / actors didn't shine in the series, but I feel he might be the character that really made the series work.