My intro to tube rolling began seven years ago when I bought a Blue Circle BC21 preamp.
I was pretty shocked by the transformation when I replaced the stock 6SN7s with WWII era VT-231s.
My favorites for the BC21 are the ones Rodman99999 suggests, the Sylvania and Ken-Rad VT-231s. Both tubes convey an enormous sense of scale while also imparting the positive qualities of this era of tubes. Refinement, dimensionality and a great midrange.
I feel the Sylvania is a bit more neutral, while the Ken-Rad seems more organic. I should say I have a preference for the black glass Ken-Rad over the clear glass version. In my set up, the black glass sounds, bigger, bolder and richer.
Godd luck!