I'm not saying she's not doing a phenomenal job. I'm just saying that I doubt she's the only person who could do a great job of it. There just isn't much demand for it, so there's hardly any basis of comparison. I just don't think it's fair to say she's the only person who could pull it off.
Thing is, I don't think anyone here is saying that except you. Most people are simply stating that she definitely deserves all the praise that's coming her way for her astounding performances and that she does, in fact, probably have to work a lot harder than most actors.
Yeah, I'm sure she's far from the only one who could've pulled it off, but she
is the only one who
did. And that's what we're talking about. While even I wouldn't say "no one deserves it more", she's definitely earned the gushing and I hope she gets her due (i.e. the little shiny statues).
You are correct though in stating there's not much room for comparison. The concept of clones is among few that can plausibly apply this method (along with parallel universes, and multiple personalities).
Battlestar Galactica is the only other notable genre manifestation of the clone trope and, ironically, it was the one instance where the kind of spectrum of characterizations you see among the OB clones would have been counter-intuitive, as they are literally carbon copy machines. Having them be too different (even the Sixes) would've broken the premise, IMO. Given the necessarily narrow range, I think Helfer convincingly gave each
significant Six their own "tick".
For parallel universes, we have Olivia/Altlivia from Fringe, where I also think Anna Torv did a great job establishing the "there but for the grace of God" differences between what were essentially two versions of the same exact person (same name, family, job, etc). Especially evident in the scenes where Otherlivia was pretending (badly) to be our Olivia. I always got a kick out of Altlivia's very distinctive walk.
Although it's a far different beast, Toni Collette definitely did her thing in United States of Tara (for the multiple personalities example). It's nowhere near as heavy a load (being mostly dramedy) and most of the personas were exaggerated (as often happens with real DID), but it stands on equal grounds in terms of overall quality.
Again, not a lot of examples, but quality to be found in them all. And all eclipsed by Maslany, IMHO.