A couple times a year I record concerts for the Russian Chamber Chorus of New York. They are off-the-charts good. The musicality, the detail, the sheer emotion of the work is amazing. And look, you don't have to take my word for it, even the New Yorker calls them "splendid"*
And it's not just the musical brilliance of their maestro, Nikolai Kachanov, although that's a massive part of it.
The chorus also pull these tremendous accompanists out of their hats. For instance, Mikhail Zeiger, a pianist and composer, plays with them frequently. And he's simply off-the-charts good. He's the sort of musician that, when you hear him play, you think: "Wait, he can't be as good as I think he is."
But he is.
Another impressive feat of the Russian Chamber Chorus is that they create a new program at least twice a year. Now I don't just mean they learn new music twice a year. No, the limitation of "Russian Chorus" is a limitation of over a thousand years of music. Ha! So they'll do early music, they'll do Romantic, they'll do modern music (much of their most beautiful and interesting material has been modern -- there's a 911 Mass which will bring tears to your eyes.) Sometimes they'll mix very different periods. But it's always brand new. And that makes things more adventurous for the singers and the audience.
*I can't actually find the link right now to the New Yorker blurb. Sue me.