With all the La Bella love here, let me round out (pun intended) the discussion. La Bellas are fantastic neutral toned strings - good articulation and not too bright or dark for flats. By comparison, here are how some of the other conventional flats offerings sound to my ears:
D'Addario Chromes: top end zing fading to thump over time
Fender flats: growl that gradually fades over the course of a long time
Ernie Ball conventional steel flats: less pronounced growl that mellows nicely
Thomastik-Infeld: a very mellow, sweet tone
Optima: successor to the '60's Maxima - think Sir Macca Sgt. Pepper
Pyramid: straightforward, higher tension
Rotosound monel: some clank, very high tension for the given gauge
GHS Precision Flats: a little rattle-y when new, locks in to a deep, almost hollow tone over time.
New Ernie Ball Cobalt: brightest for flats
I have not had the pleasure of Sadowsky's yet; others think very highly of them.
As usual, YMMV.