Great question - so many of us here have experimented with different flats. My experience so far...
EB Cobalt Flats - probably the most round wound sound of flats, bold, long lasting, perfect for a TBird.
Chrome flats - bright, high mid focused, not my cup of tea - YMMV, interesting dark grey look.
Roto flats - deep, dark, plenty of thump, very traditional flats sound, very nice on a TBird.
TI Jazz flats - wonderful traditional sound, medium tension, Motown vibe, very nice on a TBird.
La Bella Black Tapes - deep, dark, woody thump, sweet mids, flat string feel, neat look on some birds.