BrickDickRick
Absolutely sublime, an essential to any MIDI enthusiast. Vanilla compatible as well.
Of course if you don't know, you should play BTSX using a source port of your choice: You can use ZDL to load up source ports and cleanly select multiple pk3s and WADs to run.
The interface of ZDL is a bit clunky but, to play BTSX all you would really need to do is drag and drop the WAD to a sourceport exe to launch it.
I recommend GZDoom for utility, Woof for authenticity, and DSDA if you're speedy boi.
Favorite track: Mystproj.