I'd argue that instruments are not necessarily meant to be played with other instruments. Of course, that's often where instruments shine, and I'm referring to those contexts mainly, with concepts like solo performances just being an example to back my ideas up.

With your movie example, I'm not asking who would be the best to watch in a film by themselves, I'm asking what actor or actress can absolutely crush a drama and a thriller and a comedy and a classic Shakespearean tragedy, being everything from a main character to a supporting role and consistently draw you in, even having so much power to be able to move you with a monologue.

The other question, the parallel question, is: what is the place of the lesser actors? What is the place of an actor who is AMAZING in one particular role, but isn't nearly as good in other roles or genres. Sure, maybe that actor from your favorite mob movie was amazing in that film, but then you watch him in a psychological thriller and find that he doesn't do as well. The performance was passable, and not BAD, but he's a bit much when you watch too many films of his in a row and you find that he really shines in one genre.

The first example is something like piano or guitar. They can be the forefront of a song, hidden in the background, or solo and it'll rarely get tired.

The second example is, say, a trumpet. Great instrument, but really shines in a few genres and when it's used too much, the sound of a trumpet might become a touch tired throughout a 50 minute album.

Of course, a lot of this argument rests on the player, but not all of the argument.