No ifs, ands or buts about it, my choice would be a 50 single ended triode. Done right, nothing makes music better IMHO. A good 50 has an absolutely room filling sound. It's harmonically rich yet nicely detailed and conveys the emotion of music better than nearly every amp I can remember listening to.
That said, Willies (drummerwill) push pull 45 is something darned special too. (oh, and my impression on the 50 come from one of WIllie's designs too).
After that, a good 6BQ5 (EL84) amp is really hard to beat.