This is really an extension of several threads already on this board, but since you asked about favorite "classical singers," I'll bite.
Classical singers, as you call them, are period-related. They fall into period categories as they perfomed classical works; in their lives, either opera or choral music. So, my take is that Buk had no "favorite classical singers." He had favorite composers, and for the most part, the composers he favored were not those who composed operatic or choral works - again, for the most part.
Let's look at the composers that are considered, by my weak hand, to be major opera composers - we have Berlioz, Bizet, Puccini, Rossini, Verdi, von Weber, Wagner and a few others. And a few score of lesser-known opera composers who made a bigger name for themselves in other genres: Beethoven, Borodin, Mussorgsky, Smetana, Tchaikovsky, ad infinitim.
The operatic works of these composers have been performed by countless orchestras and operatic singers over the past 100 years or so, which would represent which singers Buk might have heard on his transitor radio.
My point is that it's unlikely that Buk would have even cared about a particular operatic singer - if he even cared for opera. A list of his favored composers, which starts with Bach, Beethoven and Brahms, and also includes several others, such as Sibelius, is notably bereft of what we would consider to be major composers of opera. Choral work, maybe. But then again, one doesn't identify a favorite singer in choral work.
Perhaps a better question would be: Which composers of operatic or choral content did Buk prefer? Even then, my answer would likely be: "not so many."