As
@sam6550a noted, it's the
sound. 5 things at play here:
1) Personal preference. What sound involves you the most? Different strokes, different folks and all that.
2) Synergy with the amp (or component) in play. Some tubes may sound great in one amp, but maybe not so great in another.
3) Synergy with the other tubes in the component. A Tung Sol 5998 and a Melz 1578 are a beautiful marriage in many amps. That same Tung Sol with a Foton 6N8S? Not so much. Conversely, that Melz 1578 paired with, say, a Cetron 7236? Not so much also.
4) Synergy with the headphones (or loudspeakers) in play.
And once all those stars are aligned and bringing forth musical bliss?
5) We're audiophiles, and have the constant, unrelenting nag of "Can I make it even better?"