My two cents:
D'Addario Chromes: The brightest flat. Almost as bright as a roundwound with a fuller fundamental. Tension is pretty high on these, and the finish is shiny, making them a bit "sticky". Buy the extra long set for through-body stringing. Blue silk.
Sadowsky Flats: Made by LaBella, but different from LaBella's normal 760 series flats. Brighter, more flexible, but they mellow out after time. These retain some brightness after a while, but not nearly as much as Chromes. The B string is taper core on the Sadowsky flats. Do not run these through-body. Silver silk.
LaBella 760 series: I've used both light and standard gauge. These are super smooth and shiny, thumpier than both Chromes and Sadowskys. They are very even sounding strings, bright at first but quickly mellowing out to a very traditional sounding flat. They are great strings, but not as versatile as the Sadowsky strings, and nowhere near close to round wounds like the Chromes. If you need through-body stringing you have to order them that way. Black silk.
DR Legend Flats: Bright highs, thumpy lows, tension is not as "even" across all the strings. Very smooth to the touch. Great strings, but I didn't like them enough to want to use them again. Yellow silk.
GHS Precision Flats: These strings are awesome. They have a percussive attack to the front of the note that the others just don't have. They are very dull after they settle. The highs are attenuated, but there is an upper mid presence that makes them growl. Nothing like roundwounds, but if you're looking for a thumpy sound, this is it. Very smooth finish, tension is a little higher than the others, maybe not as much as Chromes. The price make these the best buy. Purple silk.
For me, they're all great. They each serve a purpose or a bass.